Crossbar spinal prosthesis having a modular design and related implantation methods

ABSTRACT

Modular spinal prosthesis having one of both of adaptable and configurable components are provided. The modular spinal prosthesis described herein provide an artificial articular configuration to replace damaged, worn or otherwise removed spinal facet elements.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/948,973, filed Nov. 30, 2007, which is a continuation of U.S. patentapplication Ser. No. 10/973,939, filed Oct. 25, 2004, which is acontinuation-in-part of and claims the benefit of commonly assigned U.S.patent application Ser. No. 10/831,657, filed Apr. 22, 2004, all ofwhich are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention generally relates to devices and surgical methodsfor the treatment of various types of spinal pathologies. Morespecifically, the present invention is directed to several differenttypes of highly configurable and anatomically adaptable spinal jointreplacement prostheses and surgical procedures for performing spinaljoint replacements.

BACKGROUND OF THE INVENTION

The human spinal column 10, as shown in FIG. 1, is comprised of a seriesof thirty-three stacked vertebrae 12 divided into five regions. Thecervical region includes seven vertebrae, known as C1-C7. The thoracicregion includes twelve vertebrae, known as T1-T12. The lumbar regioncontains five vertebrae, known as L1-L5. The sacral region is comprisedof five fused vertebrae, known as S1-S5, while the coccygeal regioncontains four fused vertebrae, known as Co1-Co4.

FIG. 2 depicts a superior plan view of a normal human lumbar vertebra12. Although human lumbar vertebrae vary somewhat according to location,they share many common features. Each vertebra 12 includes a vertebralbody 14. Two short boney protrusions, the pedicles 16, extend backwardfrom each side of the vertebral body 14 to form a vertebral arch 18.

At the posterior end of each pedicle 16, the vertebral arch 18 flaresout into broad plates of bone known as the laminae 20. The laminae 20fuse with each other to form a spinous process 22. The spinous process22 serves for muscle and ligamentous attachment. A smooth transitionfrom the pedicles 16 to the laminae 20 is interrupted by the formationof a series of processes.

Two transverse processes 24 thrust out laterally, one on each side, fromthe junction of the pedicle 16 with the lamina 20. The transverseprocesses 24 serve as levers for the attachment of muscles to thevertebrae 12. Four articular processes, two superior 26 and two inferior28, also rise from the junctions of the pedicles 16 and the laminae 20.The superior articular processes 26 are sharp oval plates of bone risingupward on each side of the vertebrae, while the inferior processes 28are oval plates of bone that jut downward on each side.

The superior and inferior articular processes 26 and 28 each have anatural bony structure known as a facet. The superior articular facet 30faces medially upward, while the inferior articular facet 31 (see FIG.3) faces laterally downward. When adjacent vertebrae 12 are aligned, thefacets 30 and 31, capped with a smooth articular cartilage andencapsulated by ligaments, interlock to form a facet joint 32, alsoknown as a zygapophyseal joint.

The facet joint 32 is composed of a superior facet and an inferiorfacet. The superior facet is formed by the vertebral level below thejoint 32, and the inferior facet is formed by the vertebral level abovethe joint 32. For example, in the L4-L5 facet joint, the superior facetof the joint 32 is formed by bony structure on the L5 vertebra (i.e., asuperior articular surface and supporting bone 26 on the L5 vertebra),and the inferior facet of the joint 32 is formed by bony structure onthe L4 vertebra (i.e., an inferior articular surface and supporting bone28 on the L4 vertebra).

An intervertebral disc 34 between each adjacent vertebra 12 permitsgliding movement between the vertebrae 12. The structure and alignmentof the vertebrae 12 thus permit a range of movement of the vertebrae 12relative to each other.

Back pain, particularly in the “small of the back” or lumbosacral(L4-S1) region, is a common ailment. In many cases, the pain severelylimits a person's functional ability and quality of life. Such pain canresult from a variety of spinal pathologies.

Through disease or injury, the laminae, spinous process, articularprocesses, or facets of one or more vertebral bodies can become damaged,such that the vertebrae no longer articulate or properly align with eachother. This can result in an undesired anatomy, loss of mobility, andpain or discomfort.

For example, the vertebral facet joints can be damaged by eithertraumatic injury or by various disease processes. These diseaseprocesses include osteoarthritis, ankylosing spondylolysis, anddegenerative spondylolisthesis. The damage to the facet joints oftenresults in pressure on nerves, also called “pinched” nerves, or nervecompression or impingement. The result is pain, misaligned anatomy, anda corresponding loss of mobility. Pressure on nerves can also occurwithout facet joint pathology, e.g., a herniated disc.

One type of conventional treatment of facet joint pathology is spinalstabilization, also known as intervertebral stabilization.Intervertebral stabilization prevents relative motion between thevertebrae. By preventing movement, pain can be reduced. Stabilizationcan be accomplished by various methods. One method of stabilization isspinal fusion. Another method of stabilization is fixation of any numberof vertebrae to stabilize and prevent movement of the vertebrae.

Another type of conventional treatment is decompressive laminectomy.This procedure involves excision of part or all of the laminae and othertissues to relieve compression of nerves.

These traditional treatments are subject to a variety of limitations andvarying success rates. None of the described treatments, however, putsthe spine in proper alignment or returns the spine to a desired anatomyor biomechanical functionality. In addition, stabilization techniqueshold the vertebrae in a fixed position thereby limiting a person'smobility and can compromise adjacent structures as well.

Prostheses, systems, and methods exist which can maintain more spinalbiomechanical functionality than the above discussed methods and systemsand overcome many of the problems and disadvantages associated withtraditional treatments for spine pathologies. One example of suchprosthesis is shown in FIG. 4. FIG. 4 shows an artificial cephalad andcaudal facet joint prosthesis 36 and 50 for replacing a natural facetjoint. Cephalad joint prosthesis 36 replaces the inferior facet of anatural facet joint. Cephalad prosthesis 36 has a bearing element 38with a bearing surface 40. Caudal joint prosthesis 50 replaces thesuperior facet of a natural facet joint. Caudal prosthesis 50 has abearing element 52 with a bearing surface 54. Conventional fixationelements 56 attach cephalad and caudal facet joint prostheses 36 and 50to a vertebra in an orientation and position that places bearing surface40 in approximately the same location as the natural facet joint surfacethe prosthesis replaces. The prosthesis may also be placed in a locationother than the natural facet joint location.

The prosthesis illustrated in FIG. 4 addresses the immediate problem offacet joint degeneration and restores biomechanical motion. However,this exemplary prosthesis, in addition to others, would benefit fromdesign features having more modular components or a design that lendsitself to attaching to the spinal bone in a greater variety oforientations and/or locations. In general, the desire for these kinds ofdesign changes are referred to generally as prosthesis customization.

Prosthesis customization to patient specific disease state and anatomyare among the challenges faced when implanting a prosthesis. Thechallenges are amplified in the implantation of spinal prostheses thatrestore facet biomechanical function and vertebral body motion. Currentprostheses designs have not provided prosthesis systems having modulardesigns that are configurable and adaptable to patient specific diseasestate and anatomy.

There is a need in the field for prostheses and prosthetic systemshaving configurable designs and that are adaptable to a wide variety ofspinal anatomy and disease states to replace injured and/or diseasedfacet joints, which cause, or are a result of, various spinal diseases.There is also a need for surgical methods to install such prostheses.Additionally, there is also a need for prostheses and prosthetic systemsto replace spinal fusion procedures

SUMMARY OF THE INVENTION

In one embodiment of the present invention there is provided a facetjoint prosthesis to replace, on a vertebral body, a portion of a naturalfacet joint having a support component sized to span a portion of thevertebral body and adapted to receive a pair of prosthetic facetelements; and a pair of prosthetic facet elements positionable relativeto the support component to replace a portion of a natural facet joint.In a further embodiment the support component is sized to span a portionof a vertebral body is between a left lamina and a right lamina orbetween the left pedicle and the right pedicle. In still furtherembodiments, there is a kit comprising a plurality of support componentshaving different lengths. In another embodiment, the support componentis further adapted to have an adjustable width. In yet anotherembodiment, the support component is secured to the vertebral body, andin another, the support component is secured to an adjacent vertebralbody. In yet another alternative embodiment, the prosthetic facetelements are positioned relative to the support component to provide asymmetric anatomical solution and/or an asymmetrical anatomicalsolution.

In still another embodiment, the support component has an openingadapted to receive the prosthetic facet elements. In another embodiment,the prosthetic facet elements are slideable along the width of thesupport component, the prosthetic facet elements may be fixed in apre-ordained position medial of the typical a typical anatomic locationand/or the prosthetic facet elements may be fixed in a pre-ordainedposition lateral of the typical a typical anatomic location. In anotherembodiment, the ends of the support component are adapted to receive anopening in each of the pair of prosthetic facet elements. In anotherembodiment, the pair of prosthetic facet elements is selected from aplurality of prosthetic facet elements each having an opening with adifferent depth. In another embodiment, the facet joint prosthetic facetaccording to claim 1 further comprising evenly distributing the weighton the vertebral body using the support component. In anotherembodiment, the pair of prosthetic facet elements are caudal facetelements. In another embodiment, the pair of prosthetic facet elementsare cephalad facet elements.

In another alternative embodiment, there is provided an adaptable spinalfacet joint prosthesis, having a crossbar having a first end and asecond end; a pair of cephalad prosthesis elements each having a boneengaging end and an end adapted to couple to the crossbar; and a pair ofcaudal prosthesis elements each having a surface adapted to receive acrossbar end and a fixation element. In one embodiment, the distancebetween the crossbar first end and second end is adjustable. In anotheralternative embodiment, the bone engaging end of at least one of thepair of cephalad prosthesis elements is disengagably coupled to the atleast one of the pair of cephalad prosthesis elements. In anotherembodiment, at least one of the pair of cephalad prosthesis elements orat least one of the pair of caudal prosthesis elements comprises ananti-rotation feature. In another alternative embodiment, the heightabove the crossbar of a part of a cephalad prosthesis element may beadjusted by moving the cephalad prosthesis element relative to thecrossbar cephalad prosthesis portion engaging portion. In anotheralternative embodiment, the crossbar mount posterior height is less thanthe posterior height of an adjacent spinous process when the adaptablespinal facet joint is implanted in a body.

In yet another alternative embodiment, there is provided a spinalprosthesis, comprising: a first cephalad prosthesis element and a secondcephalad prosthesis element; a first caudal prosthesis and a secondcaudal prosthesis; and a crossbar element connected to the first andsecond cephalad prosthesis elements, the crossbar element having a firstend in contact with the first caudal prosthesis and a second end incontact with the second prosthesis wherein at least one of the firstcephalad prosthesis element, the second cephalad prosthesis element, thecaudal prosthesis, the second caudal prosthesis, and the crossbarelement having a configurable portion.

In another embodiment, there is provided a spinal prosthesis, comprisinga pair of cephalad prosthesis members each comprising a distal end forsecuring to a portion of the spine and a proximal end comprising abearing element; a pair of caudal prosthesis members each comprising afixation element for securing to a portion of a spine and a bearingelement adapting to engage the cephalad prosthesis member bearingelement; and a crossbar connected between the cephalad prosthesismembers.

In another embodiment, there is provided an adaptable spinal prosthesis,comprising a pair of cephalad elements connected to act in unison with apair of cephalad arms, each of said cephalad arms comprising a proximalend, a distal end and an elbow between the proximal end, and a pair ofcaudal bearing elements adapted to engage with the pair of cephaladbearing elements.

In yet another embodiment, there is provided a caudal bearing of aspinal prosthesis, comprising a caudal bearing element having a firstsurface adapted to engage a cephalad bearing and a second surfaceadapted to engage the fixation element; and a fixation element having apreconfigured surface adapted to engage with the second surface wherebywhen the preconfigured surface is engaged with the second surface thefirst surface maintains an orientation to engage a cephalad bearing andthe orientation of the fixation element relative to the caudal bearingelement is changed to a desired orientation.

In another alternative embodiment, there is provided a spinal prosthesishaving a crossbar having a first end and a second end; a pair ofcephalad prosthesis elements having a first end for engaging a vertebraeand a second end; a pair of caudal prosthesis elements each having asurface to slidably engage a crossbar end; and a single crossbar mountfor securing the second end of each of the pair of cephalad prosthesiselements to the crossbar.

In yet another embodiment, there is provided a crossbar that isadaptable and configured for placement joining two cephalad elements, oralternatively, two caudal elements. Additional crossbar embodimentsprovide different attachment mechanism and locations between theelements. Moreover, additional embodiments provide adaptability of oneor more of one or more cephalad elements, one or more caudal elements,and/or one or more crossbar elements.

In another embodiment, there is provided a modular spinal prosthesis kitand an associated surgical method of selecting from the modular spinalprosthesis kit configurable prosthesis elements that, separately and incombination, provide an adaptable spinal prosthesis corresponding to theprosthetic needs of the patient. The kit provides a variety of varioussized cephalad and caudal prosthesis as well as various crossbars. Themethod includes selecting components from the kit having the desiredsize, angular orientation and anatomical orientation that correspond tothe prosthetic needs of the patient. In additional embodiments, there isprovided a method of adapting a prosthesis to an individual's anatomywherein the adaptability achieved by selecting from a subset ofdifferent sizes and configurations of prosthetic components.

In yet another embodiment, there is provided a method of adapting aspinal prosthesis by selecting the configuration of a prosthesis basedin part on the resulting anatomical features of a patient post-resectionor post facetectomy. The various adaptable and configurable prosthesisform a modular prosthesis system containing a number of differentcomponent configurations and orientations that, depending on diseasestate at a particular site, recision of a portion of the vertebrae/facetincluding method to form a surface for mounting the prosthesis, based onthe surface geometry created and the disease state/anatomy, selectableprosthesis such as a caudal, a cephalad and/or a crossbar element arechosen to replace the removed portion of the spine/facet joint.

In yet another embodiment there is provided a crossbar mount thatutilizes compression fittings. In another alternative embodiment, thereis provided a crossbar mount having a top cap configured to engage withvariable depth fittings on the mount body.

In another embodiment, there are provided several alternative cephaladcomponents having modular, configurable and adaptable features includingbut not limited to arm length, tip length, surface texture and crossbarengagement end and bone engagement end.

In another embodiment, there are provided several alternative caudalcomponents having modular, adaptable and configurable features includingbut not limited to stem length, inclusion of anti-rotation elements,caudal bearing angle adjustments, caudal bearing shape, size andfittings.

In another embodiment, there are provided several alternative crossbarcomponents having modular, adaptable and configurable features includingbut not limited to crossbars of fixed length, adjustable length,spherical bearings, non-spherical bearings, crossbar mount engagementconfigurations, cylindrically shaped crossbars, elongate crossbarshaving non-circular cross sections, including crossbar mount designsunique to engaging across a crossbar and a cephalad arm and in someembodiments the use of a polyaxial type connector used in combinationwith a crossbar mount joining a crossbar and a cephalad arm or in otheruses in the context of modular, adaptable and configurable prosthesis.

In another alternative embodiment, a modular spinal prosthesis isadapted to an individual anatomy by selecting and positioning the one ormore caudal elements and then based on the caudal component placementand the existing anatomy, select crossbar and cephalad components toconform to the caudal prosthesis component placement. In anotheralternative embodiment, a modular spinal prosthesis is adapted to anindividual anatomy by selecting and positioning the one or more cephaladelements and then based on the cephalad component placement and theexisting anatomy, select crossbar and caudal components to conform tothe cephalad prosthesis component placement.

In additional alternative embodiments, there are provided differentcomponents, methods and configurations to provide improved tissueshielding capabilities, such as for example, placing the selected orbasing the selection of the modular components on reducing theoccurrence of tissue being caught in the prosthesis. In one specificembodiment, the relative positions are modified such as by reversing thecaudal and the cephalad bearings to protect tissue from getting caughtin the contacting arms.

These and other features and advantages of the inventions are set forthin the following description and drawings, as well as in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a lateral elevation view of a normal human spinal column;

FIG. 2 is a superior view of a normal human lumbar vertebra;

FIG. 3 is a lateral elevation view of vertebral facet joint;

FIG. 4 is a perspective view of a spinal prosthesis;

FIG. 5 is a perspective view of the anatomical planes of the human body;

FIG. 6 is an perspective view of an embodiment of a modular spinalprosthesis of the present invention;

FIGS. 7-8F are various views of several alternative embodiments of acaudal prosthesis.

FIGS. 9-11B are various views alternative crossbar embodiments;

FIGS. 12A-13B are various views of a caudal bearing cup embodiment;

FIG. 14 is a perspective view of an embodiment of a cephalad prosthesiselement;

FIG. 15 is an embodiment of an assembled modular prosthesis of thepresent invention;

FIGS. 16A-16B illustrate the internal components of an embodiment of acrossbar mount.

FIGS. 16C-16F illustrate an alternative embodiment of a crossbar mount;

FIG. 17 is a posterior view of the cephalad portion of an embodiment ofa modular prosthesis;

FIG. 18 illustrates a kit embodiment of a modular prosthesis of thepresent invention;

FIG. 19 is a flow chart of an embodiment of a surgical method;

FIG. 20-24 illustrate one method of implanting an embodiment of amodular prosthesis of the present invention;

FIG. 25 is a section view of the implanted modular prosthesis of FIG.24;

FIG. 25A is a section view of the caudal portion of the implantedprosthesis of FIG. 25;

FIG. 26A-26B illustrate an alternative embodiment of a modularprosthesis is an alternative crossbar mount;

FIG. 27A-27B illustrate two side views of the crossbar mount of FIGS.26A and 26B;

FIGS. 28-29 illustrate a method for implanting the prosthesis of FIGS.26A-B;

FIGS. 30A and 30B illustrate alternative crossbar mount embodiments;

FIGS. 31A-31E illustrate alternative crossbar embodiments that join thecephalad arms;

FIGS. 32A-32D illustrate alternative crossbar embodiments that join thecephalad bearings;

FIGS. 33A-36B illustrate various views of fixation members having ofanti-rotations features; and

FIG. 36C illustrates an embodiment of cephalad arms having anti-rotationfeatures and a crossbar.

The invention may be embodied in several forms without departing fromits spirit or characteristics. The scope of the invention is defined bythe appended claims, rather than in the specific embodiments precedingthem.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention provide modular spinal prosthesisthat are configurable and/or adaptable prostheses, systems, and methodsdesigned to replace natural facet joints and, in some embodiments, partof the lamina at virtually all spinal levels including L1-L2, L2-L3,L3-L4, L4-L5, L5-S1, T11-T12, and T12-L1, using attachment mechanismsfor securing the prostheses to the vertebrae. The prostheses, systems,and methods help establish a desired anatomy to a spine and return adesired range of mobility to an individual. The prostheses, systems, andmethods also help lessen or alleviate spinal pain by relieving thesource nerve compression or impingement.

For the sake of description herein, the prostheses that embody featuresof the invention are identified as either “cephalad” or “caudal” withrelation to the portion of a given natural facet joint they replace. Aspreviously described, a natural facet joint, such as facet joint 32(FIG. 3), has a superior facet and an inferior facet. In anatomicalterms, the superior facet of the joint is formed by the vertebral levelbelow the joint, which can thus be called the “caudal” portion of thefacet joint because it is closer to the feet of the person. The inferiorfacet of the facet joint is formed by the vertebral level above thejoint, which can thus be called the “cephalad” portion of the facetjoint because it is closer to the head of the person. Thus, a prosthesisthat, in use, replaces the caudal portion of a natural facet joint(i.e., the superior facet) will be called a “caudal” prosthesis.Likewise, a prosthesis that, in use, replaces the cephalad portion of anatural facet joint (i.e., the inferior facet) will be called a“cephalad” prosthesis.

When the processes on one side of a vertebral body are differentlyspaced from those on the other side of the same body, the prostheses oneach side would desirably be of differing sizes as well. Moreover, it isoften difficult and/or impossible for a surgeon to determine the precisesize and/or shape necessary for a prosthesis until the surgical site hasactually been prepared for receiving the prosthesis. In such case, thesurgeon typically needs a family of prostheses possessing differingsizes and/or shapes immediately available during the surgery. Thesurgeon cannot wait for a custom-made device to be created during thesurgery. In view of this need, embodiments of the spinal prosthesis ofthe present invention are modular designs that are either or bothconfigurable and adaptable. Additionally, the various embodimentsdisclosed herein may also be formed into a “kit” of modular componentsthat can be assembled in situ to create a custom prosthesis.

Configurable refers to the modular design of a prosthesis. For example,a configurable modular prosthesis design allows for individualcomponents to be selected from a range of different sizes and utilizedwithin a modular prosthesis. One example of size is to provide caudaland cephalad stems of various lengths. A modular prosthesis designallows for individual components to be selected for different functionalcharacteristics as well. One example of function is to provide stemshaving different surface features and/or textures to provideanti-rotation capability. Other examples of the configurability ofmodular prosthesis of the present invention as described in greaterdetail below.

Adaptable refers to the capacity of embodiments of the modularprosthesis of the present invention to select and position configurablecomponents such that the resulting spinal prosthesis will conform to aspecific anatomy or desired surgical outcome. The adaptable aspect ofembodiments of the present invention provide the surgeon withcustomization options during the implantation procedure. It is theadaptability of the present prosthesis systems that also providesadjustment of the components during the implantation procedure to ensureoptimal conformity to the desired anatomical orientation or surgicaloutcome. As described in greater detail in the illustrative embodimentsthat follow, an adaptable modular prosthesis of the present inventionallows for the adjustment of various component to componentrelationships. One example of a component to component relationship isthe rotational angular relationship between a crossbar mount and thecrossbar. Other examples of the adaptability of modular prosthesis ofthe present invention as described in greater detail below.Configurability may be thought of as the selection of a particular sizeof component that together with other component size selections resultsin a “custom fit” prosthesis. Adaptability then refers to theimplantation and adjustment of the individual components within a rangeof positions in such a way as to fine tune the “custom fit” prosthesisfor an individual patient. The net result is that embodiments of themodular, configurable, adaptable spinal prosthesis of the presentinvention allow the surgeon to alter the size, orientation, andrelationship between the various components of the prosthesis to fit theparticular needs of a patient during the actual surgical procedure.

Configurability and adaptability will at times be described in relationto an anatomical plane of the body or between a plane or plane and acomponent or components. There are three anatomical planes generallyused to describe the human body: the axial plane, the sagittal plane andthe coronal plane (see FIG. 5). Various embodiments of the spinalprosthesis of the present invention may be configurable and variablewith respect to a single anatomical plane or with respect to two or moreanatomical planes. For example, a component may be described as layingwithin and having adaptability in relation to a single plane. Forexample, a stem may be positioned in a desired location relative to anaxial plane and may be moveable between a number of adaptable positionsor within a range of positions. Similarly, the various components canincorporate differing sizes and/or shapes in order to accommodatediffering patient sizes and/or anticipated loads.

FIG. 6 is an isometric view of a modular, configurable and adaptablespinal prosthesis 100 according to one embodiment of the presentinvention. The spinal prosthesis 100 is illustrated implanted intovertebral bodies 5. The main components will be introduced withreference to FIG. 6. Each of the components will then be described inturn.

The spinal prosthesis 100 includes a crossbar 105, a pair of cephaladprostheses 120 and a pair of caudal prostheses 150. In this exemplaryembodiment the superior facets are replaced by the cooperative operationof the crossbar 105, the cephalad prosthesis 120 and the adaptablecrossbar mounts 175 that join the cephalad prosthesis 120 to thecrossbar 105. The inferior facets are replaced by the caudal prosthesis150. As described in greater detail below, the components of the spinalfacet prosthesis 100 are designed to provide appropriate configurabilityand adaptability for the given disease state, patient specific anatomyand spinal level where the implant occurs.

The crossbar 105, in a first embodiment, has a first end 110 and asecond end 115. In the illustrated embodiment the crossbar 105 is a twopiece bar where the first end 110 is attached to a threaded male portion104 having threads 109. The crossbar second end 115 is attached to athreaded female portion 106 sized to receive the threads 109. As will bedescribed in greater detail below, the threaded ends allow for the widthof the crossbar to be adjusted to mate with the width between caudalbearings 150 (FIG. 9). Additional alternative embodiments of thecrossbar 105 could include a series of solid crossbars of varying widthsand/or thicknesses, or an adjustable crossbar having some form oflocking or biasing mechanism (such as a spring-loaded tensioner ordetent mechanism, etc.).

A pair of cephalad prosthesis elements 120 are also illustrated in theexemplary embodiment of the configurable and adaptable spinal prosthesis100 of the present invention. Each cephalad prosthesis element 120includes a bone engaging end 125 and an end 140 adapted to couple to thecrossbar. The cephalad end 140 adapted to engage the crossbar includesan arm 145 and an elbow 147. The cephalad end 140 is attached to thecrossbar using the crossbar mount 175. The bone engaging end 125includes a cephalad stem 130 and a distal tip 135. The cephalad stem 130and the distal tip 135 are threaded or otherwise configured to engage.(Alternatively, the distal tip 135 could be formed integrally with thecephalad stem 130, of the same or a different material as the cephaladstem 130.) The illustrated embodiment of the cephalad stem 130 hassurface features 132. Surface features 132 may be, for example, atextured surface or other surface such as, for example, surface featuresto assist in bony in-growth. Similarly, the illustrated embodiment ofthe distal tip 135 has surface features 137.

The crossbar mount 175 is a connection structure to couple the cephaladprosthesis elements 120 to the crossbar 105. In the illustratedembodiment, the crossbar mount 175 includes a cephalad arm engagingportion 172, a cross bar engaging portion 174 and a fixation element176. As will be described in greater detail below, embodiments of thecrossbar mount 175 provide adaptability between the cephalad prosthesiselements 120 and the crossbar 105 and the loading characteristics of thecrossbar ends 110,115 and the caudal prosthesis 150.

Having provided an overview of the main components of an embodiment of aconfigurable and adaptable spinal prosthesis, each of the componentswill be described in greater detail.

Caudal Prosthesis Configurability and Adaptability

A pair of caudal prosthesis elements 150 are also illustrated in theexemplary embodiment of the configurable and adaptable spinal prosthesis100 of the present invention. Each of the caudal prosthesis elements 150includes a caudal cup 151 and a fixation element 160. The caudal cup 151includes a surface 155 adapted to receive a crossbar end and a surface157 (not shown) to engage the caudal stem head engaging surface 163 (notshown). The fixation element 160 includes a caudal stem 165 and a distaltip 170. (Alternatively, the distal tip 170 can be formed integrallywith the caudal stem 165, of the same or a different material as thecaudal stem 165.) The caudal stem 165 and distal tip 170 can be threadedor otherwise configured to engage. Additionally, the caudal stem 165 andthe distal tip 170 may include textured or otherwise functional surfacefeatures 167. In some embodiments, the features on the caudal stem 165are different from the features on the distal tip 170.

The configurability and adaptability of the caudal prosthesis 150 willnow be described with reference to FIGS. 7-8F. FIG. 7 illustrates anisometric view of a caudal prosthesis element 150. The caudal prosthesiselement 150 includes a caudal cup 151 having a surface 155 adapted toreceive a crossbar end 105, 110. The caudal cup 151 also has a surface157 adapted to receive the fixation element stem head 162. The fixationelement 160 has a caudal stem 165 and a distal end or tip 170 (aspreviously noted, the tip 170 could be formed integrally with the stem165, or can be attachable to the stem 165). The surfaces of each mayinclude textures 167 that may be the same (as illustrated) or different.The textured surfaces of the caudal stem 165 and tip 170 includetextures to, for example, promote bony in growth and/or increase thestrength of the mechanical bond with fixation cement (adhesion).

The caudal fixation element 160 may be secured directly into thevertebral body, or can be attached and/or “fixed” using a supplementalfixation material such as bone cement, allograft tissue, autografttissue, adhesives, osteo-conductive materials, osteo-inductive materialsand/or bone scaffolding materials. In one embodiment, the fixationelement 160 is enhanced with a bony in-growth surface. Examples of suchsurfaces are created using sintering processes (including the use of aporous coating on the substrate of the implant) or chemical etching(Tecomet Corporation of Woburn, Mass.) which can help fix the fixationelement within a vertebra. The bony in-growth surface can cover aportion or all of the caudal stem 165 and/or the distal tip 170.

Further details of the caudal prosthesis element 150 will be describedwith reference to FIG. 8. The caudal cup 151 has a surface 157 adaptedto receive the fixation element stem head 162. The fixation element stemhead 162 has a surface 163 adapted to engage with the surface 157. Aswill be further described below, the caudal fastener 160 and caudal cup151 are first connected together, then the caudal fastener 160 issecured to the targeted vertebrae. (Of course, if desired, the caudalfastener 160 could be implanted first and then the caudal cup 151attached thereto afterwards.) Variations in the configuration andengagement of the surfaces 157, 163 therefore determine the orientationof the caudal cup 151 and the bearing surface 155. The shape andorientation of the bearing surface 155 is a factor in how the cephaladand caudal bearing elements interact and the overall performance ofvarious spinal prosthesis embodiments of the present invention.

The challenge confronted by embodiments of the caudal prosthesis is thatthe caudal stem provides at least two significant functions. First, thecaudal stem is an anchor for the caudal prosthesis portion of the spinalimplant. As an anchor, the caudal stem requires an engaging placementwith sufficient quantity and quality of spinal bone—bone which can be ofvarying quality, quantity and anatomical orientation. To meet thischallenge, caudal stems of the present invention may be provided in asufficiently large array of angular orientations, shapes, sizes andlengths to reach and sufficiently engage with the targeted spinal bone.For example, if a patient has thin lamina or is in an excessive diseasestate requiring removal of spinal bone, then the caudal stem may benefitfrom modifications to length and orientation to reach an acceptable bonemass for fixation. In a similar manner, the caudal stem should alsoresist unwanted rotation. Second, the caudal stem is the attachmentpoint for the caudal cup. Based on the desired spinal prosthesisconfiguration, there will be a desired caudal cup orientation to provideproper engagement and alignment between the caudal cup and otherprosthesis components, such as for example, a cephalad bearing.Alteration of one or both of the surfaces 157, 163 may be utilized tomake up the difference between the position and orientation of thecaudal stem after implantation or meeting the anchoring function and theposition and orientation of an attachment point for the caudal cup. Theposition and orientation of the attachment point for the caudal cupprovides an attachment point that provides the desired orientation ofthe caudal bearing surface 155.

For purposes of explaining the configurability and adaptability of thecaudal prosthesis, the caudal stem is described as varying in relationto the caudal cup. This description is used and the caudal cupembodiments that follow illustrate the caudal cup in a desiredorientation. As such, the caudal cup appears fixed and the variation andadaptability of the caudal prosthesis is apparent by the differentpositions of the caudal stem. “Variation” refers to the relationship ofthe caudal stem into the spinal bone where the stem is implanted. As aresult of disease state, anatomy and other factors, there may be only afew possible sites and/or orientations available for caudal stemimplantation. Based on the position selected/available, the caudal stemwill have a resulting orientation relative to the caudal cup.Differences, if any, between the orientation of the caudal stem head 162and the caudal cup may be accounted for through advantageous alterationand combination of the surfaces 157, 163. This aspect of caudalprosthesis configurability and adaptability provides more options toimplant fixation elements while still providing a suitable engagement toprovide a caudal bearing surface having a desired orientation. Inoperation and for a given spinal prosthesis embodiment, there is adesired orientation of the caudal cup to engage with the cephaladbearings. Caudal stem variability provides for the advantageousinsertion angle and depth of the caudal stem into the spine to providesupport of the caudal cup. While providing the proper orientation andlength (depth) of a caudal stem, the stem must also provide anattachment point for the caudal cup. In some embodiments, theorientation of the caudal cup will be fixed and the caudal stem headmust be configurable and adaptable to accommodate the proper alignmentbetween the caudal cup and stem. In other embodiments, the caudal stemwill be fixed and the desired caudal cup configurability andadaptability must be provided by the caudal cup surface or a combinationincluding alterations to the caudal stem surface 163.

The illustrated reference system indicates how variation in therelationship between the surfaces 157, 163 can result in sagittalconfigurability and adaptability. The engagement of the surfaces 157,163 may be altered to provide a positive sagittal variation (+Osag) ornegative sagittal variation (−θsag). One of the surfaces 157, 163 may bealtered to provide the entire desired sagittal variation alone or bothof the surfaces 157, 163 may be altered so that the desired sagittalvariation is provided by the combination of the altered surfaces.

In the exemplary embodiment of FIG. 8 the surface 157 of the caudal cup151 has been altered to provide the desired sagittal variability takinginto account the disposition of the caudal stem head 162 post caudalstem implantation. In each of the embodiments that follow, therelationship between the caudal cup surface 155 and the engaging surface157 differ to some meaningful degree. In addition, the engaging surface157 desirably can include sizing or features (such as a taper lock) toremain engaged with the caudal stem head 162 throughout the range ofspinal prosthesis motion and loading. In one disclosed embodiment, thisengagement is a taper lock designed to release or “unlock” only wherethe caudal cup 155 moves towards the midline of the patient relative tothe caudal stem (desirably, the presence of the cross-bar prevents thecaudal cup from unlocking in this manner under normal loadingconditions). Alternatively, the caudal stem head 162 and stem headengaging surface 163 may be modified to provide desired variation andadaptability, or a combination of different surfaces 157, 163.

In one disclosed embodiment, the various caudal cup 151 elementsincorporate geometry resulting in a selectable sagittal angle of 1°, 6°or 11° as measured between the upper endplate of the caudal vertebralbody and the longitudinal axis of the caudal stem when projected ontothe sagittal plane. In a similar manner, the various caudal stemelements incorporate geometry resulting in a selectable axial angle of10°, 20° or 30°, as measured between the midline of the vertebral bodyand the longitudinal axis of the caudal stem, as projected onto theaxial plane. Desirably, some combination of these embodiments willaccommodate approximately 95% of the patient population.

The length of the caudal fixation element 160 is also configurable. Thelength of the caudal fixation element 160 desirably determines theoverall depth (do) the fixation element 160 penetrates the spinalimplantation site when the prosthesis 100 is implanted. The overalldepth can be determined by selecting the desired stem depth (ds) and tipdepth (dt). Different stem and tip lengths are provided to ensure thatvirtually any desired overall depth is available. Alternatively, wherethe cephalad stem is of one-piece integral construction, a series ofcephalad stems having different depths, such as a set of 25, 30, 40, 45,50 and 55 mm cephalad stems, can accommodate approximately 95% of thegiven patient population. In addition, the desired diameter of thecephalad stems can include one or more of the following: 7 mm, 6.5 mm, 6mm, 5.5 mm, 5 mm, 4.5 mm, 4 mm, 3.5 mm and 3 mm diameters. The optimalsize will depend upon the anticipated loading, as well as the level(lumbar, thoracic and/or cervical) and size of the treated pedicle andvertebral bodies. As is also made clear in the embodiments that follow,the stem 165 and the tip 170 can be separately selectable componentsthat are joined using any suitable attachment method available in theprosthetic arts.

In the disclosed embodiment, the tip 170 incorporates a distal flaredend. This flared end desirably mechanically anchors the tip within thefixation material (and/or bone) of the vertebral body. Moreover, thereduced diameter of the stem adjacent the tip desirably increases thethickness of the mantle of the fixation material, further reducing theopportunity for the stem to migrate and/or the mantle to fracture andfatigue. In a similar manner, a series of scalloped regions 170A aroundthe periphery of the tip 170 and/or stem desirably reduce and or preventrotation of the cephalad stem within the mantle of fixation material.

FIG. 8A illustrates an embodiment of a caudal prosthesis element 150′having a caudal cup 151A. Caudal cup 151A includes a surface 155 adaptedto receive a crossbar end 110, 115 and an embodiment of a surface 157Ato engage with the caudal stem head engaging surface 163. In thisembodiment of the surface 157A, the surfaces 157A, 163 engage to providepositive sagittal caudal cup-stem variation and adaptability (+θ sag).This embodiment illustrates an alteration in the surface 157A to providecaudal cup-stem variation and adaptability. Note the different thicknessbetween the caudal cup surface 155 and the engaging surface 157A (FIG.8A) and the thickness between the caudal cup surface 155 and theengaging surface 157 (FIG. 8). As a result, when the caudal cup surface157A is urged into position against the caudal stem engaging surface163, the existing stem 160 deflection is taken into account in theshapes of surfaces 157, 163 so that the caudal cup 151 and surface 155will provide the desired orientation when secured to the caudal stemhead 162.

FIG. 8B illustrates an embodiment of a caudal prosthesis element 150″having a caudal cup 151B. Caudal cup 151B includes a surface 155 adaptedto receive a crossbar end 110, 115 and an embodiment of a surface 157Bto engage with the caudal stem head engaging surface 163. In thisembodiment of the surface 157B, the surfaces 157B, 163 engage to providenegative, sagittal caudal cup-stem variation and adaptability (+θ neg).This embodiment illustrates an alteration in the surface 157B to providecaudal cup-stem variation and adaptability. Note the different thicknessbetween the caudal cup surface 155 and the engaging surface 157B (FIG.8B) and the thickness between the caudal cup surface 155 and theengaging surface 157A (FIG. 8A). As a result, when the caudal cupsurface 157B is urged into position against the caudal stem engagingsurface 163, the existing stem 160 deflection is taken into account inthe shapes of surfaces 157, 163 so that the caudal cup 151 and surface155 will provide the desired orientation when secured to the caudal stemhead 162.

The variability and adaptability of the caudal prosthesis is not limitedto only sagittal variation and adaptability. Caudal prosthesis elements150′″ and 150″″ are exemplary embodiments illustrating axial variationand adaptability. FIG. 8C illustrates an embodiment of a caudalprosthesis element 150′″ having a caudal cup 151C. Caudal cup 151Cincludes a surface 155 adapted to receive a crossbar end 110, 115 and anembodiment of a surface 157C to engage with the caudal stem headengaging surface 163. In this embodiment of the surface 157C, thesurfaces 157C, 163 engage to provide axial caudal cup-stem variation andadaptability (θ axial). This embodiment illustrates an alteration in thesurface 157C to provide axial caudal cup-stem variation andadaptability. As a result, when the caudal cup surface 157C is urgedinto position against the caudal stem engaging surface 163, the existingstem 160 deflection is taken into account in the shapes of surfaces 157,163 so that the caudal cup 151 and surface 155 will provide the desiredorientation when secured to the caudal stem head 162.

FIG. 8D illustrates an embodiment of a caudal prosthesis element 150″″having a caudal cup 151D. Caudal cup 151D includes a surface 155 adaptedto receive a crossbar end 110, 115 and an embodiment of a surface 157Dto engage with the caudal stem head engaging surface 163. In thisembodiment of the surface 157D, the surfaces 157C, 163 engage to provideaxial caudal cup-stem variation and adaptability (θ axial) to a lesserdegree than that provided by the caudal prosthesis element 150′″ (FIG.8C). This embodiment illustrates an alteration in the surface 157D toprovide axial caudal cup-stem variation and adaptability. As a result,when the caudal cup surface 157D is urged into position against thecaudal stem engaging surface 163, the existing stem 160 deflection istaken into account in the shapes of surfaces 157, 163 so that the caudalcup 151 and surface 155 will provide the desired orientation whensecured to the caudal stem head 162.

By incorporating variations in the caudal stem attachment point toaccommodate sagittal anatomical variation, and incorporating variationsin the cup attachment point to accommodate axial anatomical variation,the present embodiments can accommodate over 95% of the targeted patientpopulation using a minimal number of parts or “modules.” In the instantexample, the anatomical variations in a single pedicle of the caudalvertebral body can be accommodated by only six components. As such, itis to be appreciated that the surface 157 may be modified to providecaudal cup-stem variation and adaptability in axial, sagittal andcoronal orientations and combinations thereof.

The previous embodiments have illustrated how the surface 157 may bemodified to provide the desired caudal cup-stem variability andadaptability. Caudal cup-caudal stem variability and adaptability mayalso be accomplished utilizing a caudal cup 150 with a fixed or staticengaging surface 157. In these embodiments, caudal cup-caudal stemvariability and adaptability is accomplished by altering shape andorientation of the caudal stem head 162 and engaging surface 163. Thecaudal stem head 162 and stem head engaging surface 163 may be modifiedto provide desired variation and adaptability between the caudal cup andstem in axial, sagittal and coronal orientations and combinationsthereof. Caudal stem embodiments 160′ and 160″ are exemplary embodimentsof the possible modifications available to the surface 163 on the caudalstem head 162. Caudal stem 160′ illustrates a caudal stem head 162′having an engaging surface 163′. The shape of the engaging surface 163′is such that, when engaged to an embodiment of the caudal cup, thebearing engaging surface is in a desired position. Caudal stem 160″illustrates an caudal stem head 162″ having an engaging surface 163″.The shape of the engaging surface 163″ is such that, when engaged to anembodiment of the caudal cup, the bearing engaging surface is in adesired position.

In yet another embodiment, caudal cup-caudal stem variability andadaptability is accomplished through a combination that utilizesdifferent angled surfaces on both surface 157 and surface 163. As such,one of ordinary skill will appreciate the wide variety of caudalcup-caudal stem variability and adaptability that is provided byaltering the engaging surfaces between the caudal cup 157 and caudalstem 163.

If desired, a pad or contact surface piece (not shown) that attaches tothe stem head 162 can be used to account for discrepancies (ormisalignments) in the orientation of the implanted stem and the desiredorientation of the caudal cup. In this way the caudal cup surface andthe stem head surface would be “standard” and the contact surface wouldhave one or more inclined faces to mate between and provide the desiredstem-cup orientation. This system could incorporate a color code (i.e.,blue side to stem and yellow side to caudal cup) to inform the physicianof the proper alignment of the pad to the stem and or caudal cup. In asimilar manner, alphanumerical designators could be used to denote thesize and orientation of the contact's surfaces (i.e., 3C 5S10A—indicates a 3 degree coronal tilt, 5 degree sagittal tilt and a 10degree axial tilt).

Crossbar Configurability and Adaptability

Because the distance w between the caudal cups can vary depending uponthe placement of the caudal stems which in turn varies with the anatomyof the patient, crossbar embodiments of the present invention areadaptable and configurable to accommodate a variety of different widthsusing, for example, an adjustable crossbar 105 (FIG. 9) or a one ofseveral different fixed length crossbars 105 a (FIG. 9A). The crossbaris a support member for the bearings (or cephalad facet bearings) and issized and shaped to span the distance between a portion of the vertebralbody where the modular prosthesis is to be implanted. The portion of thevertebral body may include left and right pedicles or lamina. Asdiscussed below, the spanning distance may be fixed (crossbar 105 a) oradjustable (crossbar 105). Specifically, the threaded sections 104 and106 may be adjusted relative to the threaded portion 109 to adjust thecrossbar width w (FIG. 9). Bearings may be fixed using conventionalmeans to the ends of an adjustable width crossbar (FIG. 9) or variabledepth bearings may be fixed to either an adjustable crossbar (not shown)or to one length of several fixed length crossbars (FIG. 9A). As bestseen in FIGS. 9 and 9A, embodiments of a crossbar 105, 105 a include acylindrical bar of approximately 5 mm in diameter having a first end110,110 a and a second end 115,115 a, respectively. Spherical bearings107, 107 a (preferably between 6 and 10 mm, most preferably 8 mm indiameter) are positioned at each end 110,110 a, 115, 115 a. Desirably,the bearings 110 a, 115 a are secured to the bar 105 a by a press-fit ortapered fitting or the like (this could also include various otherfastening methods, including threads, gluing, welding or the like).

Because the distance w between the caudal cups can vary depending uponthe placement of the caudal stems (which varies with the anatomy of thepatient), the crossbar 105 a will desirably be of varying widths to spanthis distance. In one embodiment, a series of crossbars having widthsfrom 37 to 67 mm (in increments of 2 or 3 mm) is provided. FIG. 9Billustrates a variety of different length crossbars (106′_(a)-106″″_(a))corresponding to a variety of different widths (w₁-w₄). FIG. 9Cillustrates a number of alternative embodiments of bearing 107 a eachwith securing holes 9 _(a) to 9 _(d) of different depth (d₁ to d₄). Asillustrated in the embodiments of FIG. 9C, the securing hole may had adepth, d, that is less than about one-half the diameter of the bearing(i.e., d₁, d₂), about one-half the diameter of the bearing (i.e., d₃) ormore than one-half the diameter of the bearing (i.e., d₄). In addition,a selection of bearings 107 a is similarly provided, the bearings havingeach having a securing hole extending at least half-way therethrough,sized to accommodate the ends of the crossbar via a press fit.Desirably, the various bearings will have varying depths to the securingholes, with one embodiment of a system having (1) one bearing set with apair of bearings having a depth of one-half the diameter of the bearing,(2) a second bearing set having the depth of one-half of the bearingplus 0.5 mm deeper, and (3) a third bearing set having the depth of onehalf of the bearing plus 1 mm deeper. By utilizing the various crossbarand bearing combinations (and not necessarily identical depth bearingson each end of the crossbar), the ultimate width of the crossbarconstruct can be chosen from a minimum of 43 mm long to a maximum of 75mm long, in one-half millimeter and/or one millimeter increments.Various embodiments of this fixed width crossbar arrangement can be seenin FIGS. 26A, 26B, 30A and 30B, in which crossbar width adaptability isaccomplished by providing crossbars having various fixed distancesbetween the ends 110, 115 and variable depth bearings.

An alternate embodiment of a crossbar 105 constructed in accordance withthe teachings of the present invention will now be described withreference to FIGS. 10 and 10A. FIG. 10 is a posterior view of a crossbar 105 in position with a pair of caudal prosthesis 150. The crossbar105 is a threaded, two piece bar where a first end 110 is attached to athreaded male portion 104 having threads 109. A second end 115 isattached to a threaded female portion 106 having threads sized toreceive the threads 109 (FIG. 10A). The threaded ends allow for theinterpedicular crossbar width (“w”) to be adjusted. The crossbar widthis adjusted until the crossbar ends 110,115 are positioned as desired incontact with the caudal cup surface 155. The interpedicular width,crossbar width “w” or distance between the ends 110, 115 is adjusted byrotating the male portion 104 relative to the female portion 106 toeither advance the threads (i.e., increase crossbar width “w”) orretreat the threads (i.e., decrease the crossbar width “w”). Cooperativethreaded portions (i.e., male and female portions) are provided in eachend to allow the width “w” to be altered. Thus, in the illustratedembodiment, the interpedicular distance w is adjustable by rotating thefirst crossbar end relative to the second bar end.

In the illustrated embodiment, the ends 110, 115 have a generallyspherical or rounded external surface 107. The external surface 107 mayhave any shape that allows for load bearing as well as needed relativemovement between the crossbar ends and the caudal cup surface 155.Moreover, the caudal cup surface 155 may also be a factor in determiningthe crossbar end external shape 107. As will be described in greaterdetail below, the caudal cup surface 155 is adapted to receive thecrossbar ends 110, 115. In addition to the interdependency between theshape of the crossbar ends and the caudal bearing surface, the materialsused to coat or form the caudal cup surface 155 and/or the crossbar endexternal surface 107 may also be selected to improve the durability andoperation of the spinal prosthesis. The caudal cup 151 and/or bearingsurface 155 and the crossbar ends 110, 105 and/or external surface 107and/or coatings placed on any of the above may be made of any materialscommonly used in the prosthetic arts, including, but not limited to,metals, ceramics, plastics, bio-resorbable polymers, titanium, titaniumalloys, tantalum, chrome-cobalt (or cobalt-chrome), surgical steel, bonyin-growth surfaces, artificial bone, uncemented surface metals orceramics, diamond, bulk metallic glasses, or a combination thereof. Thecaudal cup 151 and/or bearing surface 155 and the crossbar ends 110, 105and/or external surface 107 and/or coatings placed on any of the abovemay be the same or different material.

FIG. 10B illustrates another embodiment of a crossbar. The crossbar 105′is a two piece bar having a first end 110 that is attached to anunthreaded male portion 101. A second end 115 is attached an unthreadedfemale portion 102. The unthreaded female portion 102 is sized toreceive the unthreaded male portion 101 and house a bias element 108.The bias element 108 urges the first end and the second end apart andinto engaging contact with the caudal cup 151. A retaining ring or othersuitable retaining device (not shown) may be included to retain the biaselement 108 in place between the male and female ends. In an embodimentof a modular spinal prosthesis utilizing a crossbar 105′, a plurality ofthe crossbars 105′ are provided each having a different working width. Aworking width refers to a range of crossbar width values within whichthe bias element may outwardly urge the ends 110, 115 into engagingcontact with the caudal bearing 151 while still providing sufficientstructural strength for the crossbar 105′ to operate as a load bearingelement within the spinal prosthesis

The crossbar ends 105, 110 and the caudal cup 151 and bearing surface155 may also be any appropriate and cooperative shapes to giveappropriate support to the prosthesis bearing components, the spine andto provide the appropriate range of motion for the anatomical locationof the prosthesis. FIG. 11A illustrates an exemplary modification to theexternal surface 107 of the crossbar ends 110, 115. In the illustratedembodiment, a portion of end of crossbar outer surface 107 has beenmodified to provide altered surface 107′. Altered surface 107′ has beenadded to the crossbar ends 110, 115 to improved bearing performance ofthe ends 110, 115 against the caudal bearing surface 155. In theillustrated embodiment, a crossbar end lateral portion 107′ has beenaltered to provide an improved bearing surface with the lateral surface159. In this embodiment, the lateral surfaces 159 and the shapedcrossbar end surface 107′ are both flat. Other shapes are possible, suchas a shape that conforms to the inner surface of the caudal cup at theends of each range of motion.

In alternate embodiments, the entirety or a portion of the crossbars mayhave non-circular cross sections, including polygonal, hexagonal, oval,etc, to reduce and/or prevent rotation of the crossbar during loadingconditions, as well as to allow the crossbar to be rotated (if desired)using tools such as wrenches, etc.

Accordingly, embodiments of the crossbar may be utilized as a supportcomponent sized to span a portion of the vertebral body and adapted toreceive a pair of prosthetic facet elements. The pair of prostheticfacet elements are positionable relative to the support component toreplace a portion of a natural facet joint. Additionally, there may alsobe a kit comprising a plurality of support components having differentlengths, or alternatively, the crossbar or support element may befurther adapted to have an adjustable width. In some embodiments, thecrossbar may be secured to a vertebral body or to an adjacent vertebralbody. The crossbar or support member in conjunction with othercomponents may be used to provide symmetric and/or asymmetric anatomicalsolutions. In other embodiments, the support component has an openingadapted to receive the prosthetic facet elements, and/or the prostheticfacet elements are slideable along the width of the support component.The prosthetic facet elements may be fixed in a pre-ordained positionmedial or lateral of the typical a typical anatomic location. While thecrossbar has been illustrated in embodiments where the prosthetic facetelements are cephalad elements, embodiments of the crossbar or supportcomponent may also be used with caudal prosthetic facet elements.

Additional modification of the caudal cup are also possible in order toimprove the operation and reliability of the prosthesis through therange of spinal motion. One such modification is illustrated in FIG.11B. Caudal cup 150′ is a modified version of the caudal cup 150. Thecaudal cup 150′ includes an upper crossbar end retainer 161 and a lowercrossbar end retainer 164. The upper and lower crossbar end retainers161, 164 may optionally be provided to reduce the likelihood that thecrossbar ends 110, 115 will slide out of contact with or leave anacceptable area adjacent the caudal cup surface 155 (dislocate). In asimilar manner, the posterior surface of the caudal cup could also beclosed (not shown), thereby capturing and holding the crossbar ends 110,115 and limiting and/or preventing posterior movement of the crossbarrelative to the caudal cups. In this alternate embodiment, the caudalcups could also comprise a “clamshell” design with the lower portion(shown in FIG. 11A) and a mating shape (not shown) that clamps, bolts,clips, or bonds to the lower portion

The caudal cup 151 desirably provides a surface 155 to engage with thecrossbar ends 110, 115 and will be described with reference to bothFIGS. 12A and 12B. The surface 155 is adapted to receive a crossbar end.The surface 155 has a size, shape(s), and contour(s) that may be adaptedto allow, for example, for sliding and relative motion between thecrossbar ends 110,115 and the caudal cup 151 during relative motionbetween the treated spinal levels. As used herein, relative spinalmotion includes flexion, extension, lateral bending, axial/torsionalrotation and compound motions including combinations of the above listedtypes of motion.

The surface 155 is best illustrated with reference to FIGS. 12A and 12B.The surface 155 refers to the interior surface of the caudal cup 151that is adapted to receive and engage the cross bar ends 105, 110. Oncean adaptable spinal prosthesis embodiment of the present invention isimplanted into a portion of the spine, the forces generated between thecross bar ends 105, 110 and the caudal cup interior surface 155 willchange depending upon the relative movement (i.e., flexion, rotation,extension, etc.) between adjacent vertebrae containing the prosthesis.Force and loading profiles created in the prosthesis will also changedepending upon the type and magnitude of the movement. In addition, thecaudal cup engaging surface 155 should be configured to allow forrelative motion between the crossbar ends 105, 110 and the caudal cupengaging surface 155 while also preventing the cross bar ends 110, 105from disengaging from the caudal cup 151. The illustrated embodiment ofthe surface 155 includes: an upper edge 152, an upper bottom surface153, a lower edge 154, a lower bottom surface 156, a medial edge 158,and a lateral edge/surface 159. The size, shape, relationships betweenand relative positions of the above listed facets of the surface 155provide wide ranging options for the configurability and adaptability ofthe surface 155.

Advantageously, embodiments of the present invention provide engagingsurfaces 155 that are highly adaptable and may be configured in a numberof ways to accommodate a wide range of force and loading profiles. FIGS.25 and 25 a illustrate how the flexion angle (θ_(F)) relates to theshape and slope of the upper bottom surface 153. The flexion angle isdesirably determined relative to the upper endplate of the caudalvertebral body. A line, labeled CEP on FIG. 25, desirably runs parallelto the upper endplate of the caudal vertebral body. A line perpendicularto the CEP (extending along the longitudinal axis of the caudalvertebral body—labeled PEP) is then determined, and the flexion angle(θ_(F)) is the angle of the upper bottom surface 153 relative to thePEP. A wide variety of flexion angles may be provided by altering theslope of the upper bottom surface 153. Desirably, the flexion anglesassociated with various embodiments of the present invention would rangefrom 15 degrees to 35 degrees. More desirably, the flexion angles wouldrange from 20 degrees to 30 degrees. In the most desirable embodiment,the flexion angle is a 25 degree ramp.

It should be understood that many of the angles discussed herein aredescribed with reference to one or more two-dimensional angle measuringsystems, even though the angles themselves are actually positioned inthree-dimensional space. Accordingly, the disclosed desired anglemeasurements, when projected upon a two-dimensional reference frame, maydiffer to some degree (however slight) from the specific angles and/orangle ranges disclosed herein, depending upon the extent to which thecomponents of that angle relate to the reference frame.

A functional spine unit can be defined as the caudal and cephaladvertebral body and the interspinal disk and facet tissues (as well asconnective tissues) therebetween (effectively the upper and lowervertebral bodies and the joints therebetween). Because the naturalmotion of each functional spine unit can differ depending on the spinallevel as well as variations in the natural spinal anatomy, the desiredflexion angles can differ from unit to unit. In one disclosed embodimentfor the replacement of facet joints in the L3-L4 and/or L4-L5 levels, aflexion angle of 25° will desirably (1) allow significantfreedom-of-motion to the treated unit, thereby closely mimicking thefreedom-of-motion allowed by the original anatomy, and (2) provide forsignificant stabilization of the treated level, especially where theremoval of connective tissues and/or related structure(s) hasdestabilized the treated unit.

In alternate embodiments, additional crossbar motion may be accommodatedby altering the caudal cup width (w_(cup)) or adjusting the distancebetween the medial edge 158 and the lateral edge 159 in someembodiments. If desired, the upper edges of 152 and 154 could curve overat the top to enclose (partially or fully) the upper portion of the cup151. In other embodiments, the radius of the curve that transitionsbetween the lateral edge 159 and the upper edge 152 and the radius ofthe curve that transitions between the lateral edge 159 and the loweredge 154 may also be adjusted to accommodate the various shapes of thecrossbar end outer surface 107. In additional alternative embodiments,the medial edge 158 and lateral edge 159 are nonparallel. In otherembodiments, the medial edge 158 and the lateral edge 159 could have anactuate shape, or the cup 151 could be completely enclosed with aflexible and/or rigid cover or “cap”. Such arrangements could helpprevent dislocation of the construct and/or allow for spontaneousrelocation of the bearing surface (operatively, minimally invasively ornon-invasively).

In one alternate embodiment, once the cephalad and caudal components ofthe prosthesis have been secured to the targeted vertebral bodies, oneor more elastic compression devices or “bands” could be secured aboutthe caudal cups and bearing elements (or to the vertebral bodiesthemselves, or between other parts of the cephalad or caudal prosthesis,or any combination thereof). Properly positioned and/or tensioned, these“bands” would tend to keep the bearing surfaces and caudal cups incontact and/or close proximity, even under extreme and/or unusualloading conditions, and thus reduce and/or eliminate the opportunity forthe bearing elements to dislocate. Moreover, in the event thatdislocation of the implant did occur, the bands could prevent and/orlimit motion of the dislocated joint (by holding the bearing surfacesand caudal cups together), and thus reduce or eliminate damage to othertissues (such as the spinal cord, various other nerves and/orcirculatory/connective tissues) resulting from the dislocation. In fact,the compression of the bands might make it possible to eventually“reduce” the dislocation and/or repair the dislocated prosthesis throughexternal manipulation and/or minimally-invasive surgery. If desired, oneor more “bands” could be secured between the articulating surfaces ofthe prosthesis, or between the various arms, cups, stems and/orcross-arms of the construct elements, with varying results.

In another alternate embodiment, the compression device could comprisean elastic or pliable material surrounded by a non-elastic housing,whereby the elastic material allows various movement of the bearingsurfaces (with resistance commensurate to the flexibility of thematerial), but the non-elastic housing acts as an ultimate “stop” tomovement of the bearing surfaces relative to the caudal cup. Similarly,the “band” could comprise an elastic, non-elastic or rigid material,such as stainless steel cable, which desirably prevents relative motionof the prosthesis components beyond a certain pre-defined maximumextension/flexion.

In a disclosed embodiment, the caudal cup has a length of 11.3 mm and awidth of 8 mm. Desirably, this arrangement will allow the facetreplacement construct to move approximately 15° (between full flexionand full extension of the construct). In one embodiment, the extensionwill stop at approximately −2° and the flexion will stop atapproximately 13° (relative to the longitudinal axis of the spine). Ifdesired, the lateral wall could have a slightly medial inclination toassist in keeping the crossbar ends within the cup during extreme rangeof motion. Similarly, the implant is desirably able to accommodate atleast 7.5° lateral bending to each side.

The caudal cup 151 or the surface 155 may be formed from or coated witha material, e.g. polyethylene, polyurethane, Ultra High Molecular WeightPolyethylene (UHWMP), ceramic, or metal (as well as those materialspreviously described), which provides glide and cushioning ability forany potential contacting components, such as the crossbar ends orcephalad bearings. In one embodiment (see FIG. 12A), the surface 155 canbe formed in a gently upwardly curving shape, similar in shape to acatcher's mitt. Desirably, the caudal cup 151 can be sized to be largerthan the crossbar ends 110, 115, allowing for significant articulationand motion of the joint. In addition, the cup 151 and/or surface 155 maycomprise modular components of varying sizes, shapes and/ororientations, further increasing the adaptability and/or configurabilityof the prosthesis.

FIGS. 13A and 13B illustrate a cross bar end 110 at the extreme ends ofthe range of motion for an illustrative embodiment of the caudal cup 151and surface 155. At full flexion, the cross bar end 110 can be incontact with the upper edge 152 (FIG. 13A). At full extension, thecrossbar end 110 can contact the lower edge 154 (FIG. 13B). Dependingupon the desired range of motion, the design of the caudal cups andcrossbar ends, and the configuration of the implanted components, thecrossbar ends will desirably ride against the surface 155 throughout theentirety of the range of motion, and will only sit above and/or notcontact the caudal cup surface 155 at the extreme ends of the range ofmotion.

Cephalad Prosthesis Configurability and Adaptability

An embodiment of a cephalad prosthesis element 120 is illustrated inFIG. 14. The exemplary embodiment of the cephalad prosthesis elementincludes a bone engaging end 125, a crossbar engaging end 145 and anelbow 147 between the ends 125, 147.

Similar to the caudal stem, the bone engaging end is used as anattachment point to spinal bone and an anchor for the crossbar. The boneengaging end 125 includes a cephalad stem 130 and a distal tip 135. (Aspreviously noted, in various embodiments the distal tip may beconfigurable or may be formed integrally as part of the cephalad stem.)The length of the bone engaging end 125 in this embodiment isconfigurable. The length of the bone engaging end 125 determines theoverall depth (d_(o)) the bone engaging end 125 penetrates the spinalimplantation site when the prosthesis 100 is implanted. The overalldepth (d_(o)) is determined by selecting the desired stem depth (d_(s))and tip depth (d_(t)). Different stem and tip lengths are provided toensure that virtually any desired overall depth is available. In variousembodiments, the overall depth (d_(o)) can range from 35 mm to 55 mm (in5 millimeter increments). In one embodiment, the diameter of thecephalad stems is approximately 6.5 mm, with a minimum diameter(proximate the flared distal tip) being no less than approximately 5.5mm.

The distance from the elbow 147 to the distal tip 135 can also beconfigurable and adaptable depending upon the length of a configurabledistal tip selected to attach to a fixed length cephalad stem 130. Inone embodiment, the cephalad stem 130 has a fixed length and the distaltip 135 may be selected from a number of distal tips 135 having avariety of lengths. In this embodiment, the bone engaging end 125 lengthwill be the sum of the fixed length cephalad stem 130 and the length ofthe selected distal tip 135. Alternatively, the length of each of thecephalad stem 130 and the distal tip 135 may be configurable. In thisembodiment, the bone engaging end 125 length will be the sum of thelength of the selected cephalad stem and the length of the selecteddistal tip (i.e., 120A, B and C and 170A through 170E of FIG. 18).

In various alternate embodiments, the arm length of the cephalad element120 can be configurable. Between the crossbar engaging end 140 and theelbow 147 is the arm 145. Embodiments of the cephalad prosthesis 120 mayinclude arms of a variety of different lengths. In another embodiment,the arm length is selected such that the resulting dorsal height of theprosthesis 100, when implanted, is equal to or less than the dorsalheight of an adjacent spinous process, or can be equal to or less thanthe average dorsal heights of the immediate adjacent vertebral levels.In various embodiments, dorsal height can be measured relative to thecaudal vertebral body and/or the cephalad vertebral body, or can bemeasured with regards to an approximate average value there between. Inone embodiment, the dorsal height of the construct is not greater thanapproximately 22 mm from pedicle entry point to the most dorsal point.In an alternate embodiment, the dorsal (posterior) height of theconstruct is not greater than approximately 25 mm from pedicle entrypoint to the most dorsal point.

Another aspect of the configurability and adaptability of the cephaladelement 120 is the elbow angle (θ_(elbow)). The elbow angle (θ_(elbow))is the angle formed between the bone engaging end 125 and the crossbarengaging end 140. In the illustrated embodiment, the elbow angle isabout 90 degrees. In alternative embodiments, the elbow angle may begreater than or less than 90 degrees, or could possibly range from 60°to 100°, desirably in 5° increments. Moreover, while the arm 145 in thedisclosed embodiment is essentially straight, other embodiments couldincorporate varying arm orientations, including curved, rounded orcompound angles and/or shapes (including C or S-shapes).

The cephalad prosthesis 120 may itself be made of any joint materialscommonly used in the prosthetic arts, including, but not limited to,metals, ceramics, titanium, titanium alloys, tantalum,chrome-cobalt/cobalt-chrome, surgical steel, bony in-growth surfaces,artificial bone, uncemented surface metals or ceramics, or a combinationthereof. The bone engagement end 125 may be secured directly into avertebral body, or can be attached and/or “fixed” using a supplementalfixation material such as bone cement, allograft tissue, autografttissue, adhesives, osteo-conductive materials, osteo-inductive materialsand/or bone scaffolding materials. In one embodiment of an adaptablespinal prosthesis of the present invention, at least one bone engagementend 125 is enhanced with a bony in-growth surface. Examples of suchsurfaces are surfaces created using aggressive bead blasting, sinteringprocesses, porous coatings on substrates, or chemical etching (TecometCorporation of Woburn, Mass.) which can help fix the fixation elementwithin a vertebra. In other embodiments, the bony in-growth surface cancover a portion or all of the bone engaging end 125. In yet anotheralternative embodiment, the textured surfaces 132, 137 include a bonyin-growth surface. Textured surfaces 132 and 137 may be the same ordifferent. Either or both of the textures surfaces 132, 137 may includefeatures or surface finish to improve or assist in, for example, bonyin-growth, or bone cement adhesion.

Alternative embodiments of the present invention could include aprosthesis system having selectable elbows with a stem receiving end andan arm receiving end, arms of different lengths having an end to engagewith the elbow arm receiving end and an end to engage with the crossbar;cephalad stems having a variety of lengths and an end adapted to engagethe elbow stem receiving end and an end adapted to receive a distal tip;and distal tips having a variety of lengths and cephalad stem engagingends. In this embodiment, the starting point could be the elbow angle.Unlike the single dimension elbow angle of FIG. 14, this elbow anglewould include configurability in any one or a combination of thesagittal, axial or coronal planes. Once the spine had been prepared toreceive the prosthesis 100 and the surgeon understood the anatomicalorientation requirements of this specific patient, then an elbow havingthe proper orientation could be selected. The elbow would be selected asa bridge between the anchoring function of the bone engaging end and thecrossbar engaging function of the end 140. The elbow angle would also beselected such that, with the proper selection of arm length and stemlength, the cephalad prosthesis element would be in the desiredalignment for proper alignment and operation of the cephalad elementsand crossbar.

Once the desired configuration of the implant is determined, one or moreopenings or bores (to accommodate the anchoring stems) can be created inthe targeted vertebral bodies, and the caudal and cephalad componentsinserted. If desired, the physician can employ a trialing system orother type of measurement tool (e.g., a device that determines the sizeand orientation of the various modular components so as to provideproper alignment between the caudal cup and the cephalad attachmentpoint—caudal stem length and cup orientation, an elbow having thedesired angular relationship, a cephalad stem of the indicated lengthand an arm of the indicated length). These pieces can all be fastenedtogether and test fitted in their respective positions on the vertebralbody. If a proper fit is achieved, then the pieces are cemented orotherwise permanently joined and the cephalad stem is cemented orotherwise joined to the spinal bone.

FIG. 15 illustrates an embodiment of an assembled configurable andadaptable spinal prosthesis 100. This embodiment illustrates how thevarious components of the prosthesis may be selected and configured toaccommodate an individual's anatomy. For example, the illustratedembodiment utilizes differing caudal prosthesis. Crossbar end 110engages with a caudal prosthesis 150′ while the crossbar end 115 engageswith a caudal prosthesis 150. Both caudal prosthesis 150, 150′ havefixed length caudal stems 165. The caudal prosthesis 150′ has a caudalfastener length that is the sum of the caudal stem 165 and tip 170. Thecaudal fastener length of the caudal prosthesis 150 is longer becausethe distal end 170′ has a length longer than the distal tip 170 of thecaudal prosthesis 150′. Similarly, the cephalad prosthesis fastenershave different length or depths because the cephalad tip 135′ is longerthan the cephalad tip 135.

FIG. 15 also illustrates the inner structure of one embodiment of acrossbar mount 175. The crossbar 175, including interior components 172and 174, will be described with reference to FIGS. 15, 16A and 16B. Thecrossbar interior components 172, 174 are illustrated in phantom in FIG.15 and are illustrated in detail in FIGS. 16A and 16B. Arm-crossbar lockengaging element 179 includes a first surface 171 for engaging thecephalad arm end 140 and a second surface 173 for engaging the crossbar105. The crossbar locking element 181 includes a first surface 177 forengaging the cross bar 105 and a second surface 178 shaped to engagewith the interior contours and shape of the crossbar arm mount 175. Eachof the locking surfaces 171, 173, 177, 178 may include features, surfacetreatments or knurling to increase friction contact between the lockingsurface and the respective component. In one embodiment, the interiorcomponents comprise commercially-pure Titanium (CPTi) while the housingand set screw comprise ASTM F136 Titanium Alloy (Ti₆Al₄V).

The arm-cross bar lock 179 and the cross bar lock 181 each play a rollin providing adaptability to the prosthesis during implantation, fittingand securing the prosthesis in the desired anatomical orientation andposition. The fastener 176 is used to lock the cephalad arm and thecross bar into position relative to the crossbar 105. As the fastener176 compresses the cephalad arm end 140 into the lock element firstsurface 171, the lock element 179 in turn compresses the second surface173 onto the crossbar 105. The forces acting on the crossbar 105 urgethe cross bar 105 against the crossbar lock first surface 177 and, inturn, the crossbar lock second surface 178 into position against theinterior of the crossbar mount 175. As the fastener 176 is tightened,the cephalad arm end 140 is compressively secured in position relativeto the crossbar mount 175 between the fastener 176 and the arm-crossbarlock first surface 171. With the same securing action of the fastener176, the lateral position of the crossbar mount 175 in relation to thecross bar 105 or to the crossbar ends 110, 115 is also secured. As thefastener compresses the cephalad arm end 140, the cephalad arm end 140applies force to the arm-crossbar lock element first surface 171 that inturn urges the arm-cross bar lock second surface 173 against thecrossbar 105. The force applied to the crossbar 105 urges the crossbar105 against the crossbar lock first surface 177 and the crossbar locksecond surface 178 against the interior of the crossbar and arm mount175. Thus, using a single compressive force, the cephalad arm is securedrelative to the crossbar lock 175 and the crossbar lock is securedrelative to the crossbar 105 or crossbar ends 110, 115.

One advantage of the current embodiment is that the fastener 176 mayplace a compressive force against the cephalad arm end 140 and the othercomponents large enough to hold the components in position. This holdforce would be less than the force used to secure the components intothe final position for implantation. By utilizing a hold force less thana securing force, the prosthesis fit may be adjusted with regards toorientation and relationship between the components. Thereafter, thefastener 176 may be torqued to place a full compressive load onto theprosthesis to lock it into place. Once the full torque force is applied,the relatively softer CPTi (as compared to the harder ASTM F136 Ti ofthe housing, cross-bar and cephalad stems) of the arm-cross bar lock 179and the cross bar lock 181 will desirably deform and essentially lockand/or “cold weld” to the ASTM F 136 Titanium, locking the implant inits desired configuration.

FIGS. 16C through 16F depict various views of one alternative embodimentof a crossbar mount 175 a constructed in accordance with the teachingsof the present invention. These figures depict the fastener 176 a, thecephalad arm lock 179 a and the cross bar lock 181 a, with the housing1000 a illustrated in phantom. As with the previously describedembodiment, tightening of the fastener 176 a into the housing 1000 adesirably “locks” the prosthetic components in place within the housing1000 a.

In the embodiment of FIG. 16 c, a retaining pin 1010 a extends throughan opening 1015 a formed through the cephalad arm lock 179 a and pressfits into openings 1020 a in the housing 1000 a. Desirably, the opening1015 a is larger than the outer diameter of the pin 1010 a, allowing thecephalad arm lock 179 a to “float” within the housing 1000 a relative tothe pin, but be retained within the housing 1000 a. Moreover, thepresence of the cephalad arm lock 179 a will desirably retain thecross-bar arm lock 179 a within the housing 1000 a in its desiredposition as well.

The interplay between the various components of the cephalad prosthesismay be appreciated through reference to FIG. 17. FIG. 17 illustrates aposterior view of an embodiment of the cephalad portion of ananatomically adaptable spinal prosthesis of the present invention.Several adaptable features are presented. The cephalad arm height(h_(arm1) and h_(arm2)) may be adjusted, for example, by moving thecephalad cross bar engaging end 140 relative to the crossbar mount 175,or rotating the crossbar mount 175 about the crossbar 105. Whileillustrated as the having the same height, each of the cephalad arms maybe individually sized and selected as well as adjusted relative to thecrossbar mount 175 to provide different arm heights. As such, it ispossible in some embodiments that h_(arm1) will be a different valuethan h_(arm2). It is to be appreciated that in various embodiments, thesize of an individual cephalad arm could be adaptable depending upon,for example, selecting an arm length, a stem length and a distal endlength. Stem length and distal tip length are described earlier.Desirably, more than one housing, each housing having a differing angleβ (cephalad arm 145 relative to the crossbar mount) is provided. In oneembodiment, β crossbar mount is either 15° from normal or 35° fromnormal. Moreover, because the housing through-hole for the crossbarmount is larger than the diameter of the cross-bar, the housing can berotated approximately 20° with the cross-bar in position, therebyallowing for further variance and/or further misalignment of thecephalad arm relative to the cross-bar. Thus, in this embodiment, thetwo housings could accommodate angle β crossbar mount from 5° to 45°.The prosthesis width (w_(p)) is also variable by increasing ordecreasing the adjustable width (w_(a)) of the threaded portion 109between the threaded crossbar ends 106, 104 (in this embodiment) or bysimply choosing a different width cross-bar (in various alternateembodiments).

In summary, the illustrated embodiment of a cephalad prosthesis 100 ofthe invention is adaptable in at least four ways. First, the cephaladarm 145 and crossbar end 140 may move relative to the crossbar mount 175to vary cephalad arm height (h_(arm1) and h_(arm2)). Second, thecephalad arm 145 and crossbar end 140 may also rotate relative to thecrossbar mount 175 thereby moving the position of the distal tip 135along an arcuate pathway. Third, the crossbar mount, with or without thecephalad arm secured thereto, may move along the crossbar 105 towards oraway from, for example, the other crossbar mount 175, and/or the ends110, 115. Fourth, the crossbar width may be increased or decreased byrotating the threaded crossbar ends 104, 106.

The modular design aspects of embodiments of the present invention areillustrated in FIG. 18. FIG. 18 illustrates an embodiment of a kit 290having embodiments of the modular, configurable and adaptable componentsof the present invention. The kit 290 provides an organization for thevarious configurable and adaptable components of the spinal prosthesisembodiments of the invention. More importantly, the kit 290 provides away to organize the various components and simplify the process ofselecting, configuring and adapting a spinal prosthesis of the presentinvention. Adaptable spinal prosthesis kit 290 includes a plurality ofcomponents that may be utilized to produce an embodiment of an adaptablespinal prosthesis according to the present invention. These componentsare related to the spinal prosthesis 100 but are also generallyapplicable to the adaptable component embodiments of other spinalprosthesis embodiments.

Adaptable cephalad prosthesis embodiments 120A, 120B and 120C differ incephalad stem 130 length. The length of each stem 130 may be any lengthand the difference between the three sizes may be small or large. In oneembodiment, the small stem 120C has a cephalad stem length ofapproximately 35 mm (which can include a selection of bone-penetrationlengths of approximately 55 mm, 50 mm, 45 mm, 40 mm and/or 35 mm), themedium stem 120B has a cephalad stem length of approximately 45 mm andthe large stem 120A has a cephalad stem length of approximately 55 mm.While the illustrated embodiments have a common elbow angle ofapproximately 85°, it should be understood that alternative embodimentsmay include elbow angle as a configurable option—an exemplary selectionfor such a kit could include stems having elbow angles ranging from 60°to 100°, with the most desirable angle being approximately 85°.

Three exemplary crossbar sizes are also provided having increasing widthfrom 105A, 105B and 105C. The crossbar 105A may have, in an exemplaryembodiment, widths of approximately 37 mm, 51 mm and 67 mm, withpreferred adjustment widths of 0 to 15 mm. During the implantationprocess, the patient anatomy and the placement of the caudal cupscomprise two of the inputs used to determine the crossbar size. In mostinstances, the crossbar selected is narrower than the caudal cup spacingbut within the adjustable range for the threaded ends. Once the caudalcup is positioned, the crossbar may be placed in the cups and then finetuned for width using the threaded ends. In an alternative embodiment,individual cross-bars of set sizes (i.e., a set of crossbars of thefollowing widths: 37 mm, 39 mm, 41 mm, 43 mm, 45 mm, 47 mm, 49 mm, 51mm, 53 mm, 55 mm, 57 mm, 59 mm, 61 mm, 63 mm, 65 mm and 67 mm), withadjustable depth bearings, can be provided.

While the illustrated embodiment illustrates the distal tips 170 and 135having the same length selections, alternative embodiments providedistal tips 170 selectable from a variety of lengths that are differentfrom the selectable lengths for distal tip 135.

Caudal stem 160 adaptability is also illustrated by various angledstems. In this embodiment, angle θ_(c) changes for each of the caudalstem head 162 and the stem 160. θ_(c) in stem 160A is ranges fromapproximately 5° up to approximately 35°, in 5° increments. While theillustrated embodiment only illustrates one form of caudal stemadaptability, it is to be appreciated that each of the adaptablecharacteristics of the caudal stem (i.e., the stem angle θ_(c), theshape of the caudal stem head 162 and the shape of the caudal cupengaging surface 157) may each be used alone or in any combination toprovide caudal stem variability into any orientation sagittally,axially, coronally or combinations thereof. While the stem 160embodiments have been illustrated having the same length, it is to beappreciated that the stem 160 may also have various lengths or range oflengths as described above with regard to cephalad stem 130.

FIG. 19 is a flow chart illustrating an embodiment of a surgical method300 for implanting an embodiment of an adaptable spinal prosthesisaccording to the present invention. The surgical procedure comprisesexposing the spinous process, lamina, and facet joints at a desiredlevel of the spine using any method common to those of skill in themedical arts. Once the physician is prepared to implant the prosthesis,he/she will first estimate the amount of and remove any portions of thevertebral body (such as facet joints, lamina, processes, etc.) to allowfor prosthesis implantation (310). The prominent bone may be removedand/or rongeured using any means common in the field. The superior facetand/or lamina may also be trimmed to decompress the nerve root. A reameror any other instrument that is useful for grinding or scraping bone maybe used to ream, shape or contour the spinal bones as depicted in FIG.20 in preparation of implanting the prosthesis.

FIG. 20 illustrates a posterior view of vertebral bodies 40, 45 afterperforming a procedural bone resection, a wide decompressivelaminectomy, facetectomy and/or laminectomy to the degree determined instep 310 and discussed above. Some and/or all of the spinous process andinferior facet joints have been removed from vertebra 40 to removediseased bone, relieve pressure on nerves or other tissues, and/orcreate sufficient space for placement of an embodiment of an adaptablespinal prosthesis of the present invention. The superior facet jointshave been removed from vertebra 45 and the lamina shaped to producecaudal prosthesis receiving surfaces 74, 72. As illustrated in FIG. 21,holes are formed in the vertebra 40, 45 to prepare for cephalad andcaudal stem implantation and/or carry a trialing system (desirably fortrialing and/or sizing prosthesis prior to implantation). Caudal stemholes 52 and 54 are formed in vertebra 45 and cephalad stem holes 56, 58are formed in vertebra 40. The depth, size, and orientation of theseholes are used to determine selections in the prosthesis kit 290 andembodiments thereof.

Returning to the surgical method 300, size, select, test and set thecaudal prosthesis (step 320). As described above, the adaptability ofthe orientation and position of the caudal prosthesis may be utilized tomeet a wide variety of anatomical situations and to accommodate avariety of different adaptable prosthesis. It is to be appreciated thateach of the adaptable characteristics of the caudal prosthesisincluding, for example, the stem angle θ_(c), the shape of the caudalstem head 162 and the shape of the caudal cup engaging surface 157 andthe lengths of the caudal stem and distal end may each be used alone orin any combination to provide caudal stem variability into anyorientation sagittally, axially, coronally or combinations thereof. Thecaudal prosthesis may be configured by selecting the desired caudal stem(see stems 160A-160E in FIG. 18), distal tip (see distal tips 170A-170E)and caudal cup 151 (see FIG. 18). FIG. 22 illustrates the selectedcomponents after implantation. The caudal cups 151 are secured to caudalstems (not shown) that have been implanted into the caudal stem holes52, 54 formed in the vertebral body 45.

The caudal stem may be secured directly into the vertebral body, or canbe attached and/or “fixed” using a supplemental fixation material suchas bone cement, allograft tissue, autograft tissue, adhesives,osteo-conductive materials, osteo-inductive materials and/or bonescaffolding materials. In one embodiment, the first fixation element canbe enhanced with a bony in-growth surface, such as surfaces createdusing sintering processes or chemical etching (Tecomet Corporation ofWoburn, Mass.) which can help fix the fixation element within avertebra. As described above, the bony in-growth surface can cover allor a portion of the caudal fixation element. Desirably, the finalorientation of the caudal cups 155, 157 will be parallel (relative tothe lateral walls 159) and coplanar (with respect to the upper bottomsurfaces 153).

Returning to the method 300, size, select, test and adjust the crossbar(step 330). FIG. 23 illustrates an embodiment of a crossbar 105 inposition between the caudal cups 151. The crossbar 105 has been selectedfrom, in this exemplary embodiment, crossbars 105A, B and C in the kit290 (FIG. 18). The particular selection of crossbar 105A, B or C isbased, in part, on the distance between the caudal cups 151. Asdiscussed above, the width of the crossbar 105 may be selected initiallyto place the crossbar ends 115, 110 against the caudal cup receivingsurface 155. The crossbar width is adjusted into final position usingthe adjustable crossbar members 104, 106 and threaded portion 109.Additionally, the crossbar mounts 175 are present with the crossbar 105disposed within the crossbar engaging portion 174.

Size, select, test and set the cephalad prosthesis (340). Referringinitially to FIG. 23, the cephalad prosthesis is adapted to have thecrossbar engaging end 140 engage with the crossbar cephalad engagingportion 172 and the bone engaging end 125 (not shown) engaged within thelamina or spinal bone via holes 56,58. Within these parameters thecephalad arms are configured and adapted by selecting the desiredcephalad stem 130 length (see cephalad arms 120A-120C in FIG. 18). Inalternative embodiments, distal tip 135 length (see distal tips135A-135E in FIG. 18), elbow angle and arm length may also beconfigurable and selectable characteristics. As illustrated in FIG. 24,the cephalad arm crossbar engaging ends 140 are secured by fixationelement 176 to the cross bar mount cephalad engaging portion 172. Inaddition, the cephalad arm 145 has also secured the crossbar relative tocrossbar mount utilizing the locks 179 and 181 (not shown).

FIG. 24 also illustrates one of several advantages of the modular designof the present invention. One advantage is the independence of cephaladarm and crossbar mount adaptability. Note that the cephalad arm end 140in the crossbar mount adjacent the end 115 extends significantly beyondthe crossbar mount 175 while the cephalad arm end 140 in the crossbarmount adjacent the end 110 does not extend significantly beyond crossbarmount 175. Another advantage is the independence of the cephaladcomponents. Each cephalad arm 145 may be separately adjusted to bestaccommodate the anatomical situation of the patient as well as thecrossbar position and loading parameters. As illustrated, the cephaladarm adjacent end 115 is arranged differently within the prosthesis 100than the cephalad arm adjacent 110.

If desired, a series of clamps or rigs (not shown) can be used to holdeither or both of the cephalad or caudal prosthesis (or their trialinganalogs) in place during the sizing and/or testing phases and/or whilethe cement or other fixation material cures.

FIG. 25 is a section view of a portion of the spine having 4 vertebralbodies. Vertebral bodies 30 are unmodified while vertebral bodies 40, 45have been altered by the surgical techniques described with regard tosurgical method 300 to implant an embodiment of the adaptable spinalprosthesis 100.

FIG. 25A depicts the desired placement for one embodiment of a caudalcup constructed in accordance with the teachings of the presentinvention. In this embodiment, the upper endplate 45 a of the caudalvertebral body is utilized as a guide for proper placement of the caudalcup. As previously noted, a line, labeled CEP on FIG. 25A, desirablyruns parallel to the upper endplate 45 a of the caudal vertebral body(which can be visualized fluoroscopically, or via minimally-invasive oropen visualization). A line perpendicular to the CEP (extending alongthe longitudinal axis of the caudal vertebral body—labeled PEP) is thendetermined, and the flexion angle (θ_(F)) is the angle of the upperbottom surface 153 of the caudal cup relative to the PEP. Desirably, thephysician will implant and position the caudal cup such that the upperbottom surface 153 of the caudal cup is approximately 25° posterior fromthe PEP (such that the surface is located approximately 115° from theCEP). This desired position will (1) maximize the flexibility of theprosthesis, allowing for maximal proper flexion and extension of thejoint surfaces, and (2) provide a proportional amount of stability tothe prosthesis to account for removal of any connective tissues thathave occurred due to the implantation of the prosthesis as well as anyother surgical procedures impacting the connective tissues of thetreated area.

It should be understood that the angulation disclosed in this embodiment(approximately 25°) is desirably suited for replacement of the caudalfacet joints of the L4 or L5 levels of the spine. Replacement of caudalfacet joints in other levels of the spine might necessitate othervarying angulations, as well as other orientations of the caudal and/orcephalad joint surfaces to accommodate torsional movement, flexion andextension, and/or lateral bending. In addition, depending upon theactual anatomy of the L4 or L5 levels, as well as the anatomy ofadjacent levels, different angulation and/or orientation of the facetreplacement prosthesis (other than that described herein) may bedesired.

An inferior and posterior view of an embodiment of an adaptable spinalprosthesis 200 of the present invention are illustrated in FIGS. 26A and26B. This embodiment of the adaptable spinal facet joint prosthesis 200includes a crossbar 205 and a pair of cephalad prosthesis elements 220coupled to a crossbar mount 275. The crossbar 205 has two ends 210, 215engaged with a pair of caudal prosthesis elements 150. The adaptableprosthesis 200 has several features in common with the earlier describedadaptable spinal prosthesis 100 and these components are similar to theabove description. In the illustrated embodiments the cephalad boneengaging end 125 has been generalized and the caudal stems omitted forclarity.

The illustrated embodiment of the crossbar 205 has a first end 210, asecond end 215 and a plurality of indexing features 206 along a portionof the outer surface. The indexing features 206 cooperatively engagewith features 276 in the crossbar mount 275 to provide variable lateralalignment capability for the crossbar mount 275 relative to the crossbar205. The plurality of indexing features may be in sections, two areillustrated in FIG. 26A or the indexing features 206 may be spaced alongthe entire or a substantial portion of the width of crossbar 205. Theillustrated embodiment of the indexing features 206 are alignedorthogonal to the width of the crossbar 205. Other angular relationshipsare possible and are within the scope of the invention. For example, theindexing features may form a lateral angle of 0 to 45 degrees relativeto a line orthogonal to the width of the crossbar 205 measured betweenthe two ends 210, 215. The illustrated embodiment of the crossbar 205has a fixed width between ends 210, 215. Accordingly, crossbar 205 maybe provided in a variety of different, fixed widths in order to achievethe adaptability advantages of the present invention. Alternativeembodiments of crossbar 205 may include, for example, any of theadjustable width configurations described above such as threaded orslidably engaged (desirably incorporating a locking feature) crossbarpieces.

In the illustrated embodiment, there are provided a pair of cephaladprosthesis 220 having an end 240 adapted to engage the crossbar mount275, an arm 245, an elbow 147 and a bone engaging end 125. The end 240includes features 242 along the arm 245 for engaging with the outersurface of the crossbar 205. If desired, the crossbar could have acomplementary feature to engage with feature 242. In the illustratedembodiment, the features 242 are threads. Other features such asknurling, barbs, surface roughing or other surface treatment or finishto increase the hold between the cephalad arm and the crossbar may beused. Similarly, the crossbar mount 275 could incorporate a triangular,square or other geometric shaped opening (not shown) to engage acomplimentary surface (not shown) on the crossbar to reduce and/oreliminate rotation of the crossbar under loading conditions, if desired.

An exemplary embodiment of the crossbar mount 275 is illustrated inFIGS. 27A and 27B. The exemplary embodiment of the crossbar mount 275includes a housing 277, a cephalad arm engaging portion 272, and acrossbar engaging portion 274. FIG. 27A is a view of the crossbar mount275 along the cephalad arm engaging portion 272. The housing 277includes a ridge 278 that engages with the threads of interior threadedcap 280 (see FIG. 26A). The cephalad arm portion 272 is sized and shapedto engage with the cephalad arm crossbar engaging end 240. FIG. 27B is aview of the crossbar mount 275 along the crossbar engaging portion 274.The crossbar engaging portion 274 is sized and shaped to engage withcrossbar 205. Within the crossbar engaging portion 274 there is at leastone complementary indexing feature 276. Indexing feature 276 is sizedand shaped to form a cooperative mating with the crossbar indexingfeature or features 206. While the indexing features 276 are illustratedas orthogonal to the crossbar 205 other angular orientations arepossible as discussed above with regard to crossbar indexing features206.

Returning to FIG. 26A, an internally threaded cap 280 and set screw 282are used to secure the cephalad arms, crossbar mount and crossbar intothe desired position. The threaded cap 280 is secured to the housing 277using ridge 278 once the cephalad arm has been positioned within thecephalad arm engaging portion 272 and the crossbar mount features 276are engaged with the desired crossbar features 206. As the cap 280advances, the cap 280 engages the cephalad arm 245 and urges thefeatures 242 into engaging contact with the crossbar 205. At the sametime, the housing 277 urges the indexing features 276 into contact withthe crossbar indexing feature(s) 206. The cap 280 is tightened to adesired degree (and can include a breakaway feature to obtain a desiredloading of the crossbar) and then secured with the set screw 282, ifdesired

The modular prosthesis kit 290 (FIG. 18) may also be modified toaccommodate embodiments of the adaptable spinal prosthesis 200. Forexample, the cross bar portion could include a plurality of crossbar 205embodiments each having a different width. In addition, the crossbarmount 175 could also be modified to include the engagement elements 276in the desired orientation. Also, the cephalad arms could be modified toinclude the desired embodiment of features 242.

Returning to FIG. 19, which is a flow chart illustrating one embodimentof a surgical method 300 for implanting an embodiment of an adaptablespinal prosthesis according to the present invention. The method 300 wasdescribed above with regard to one embodiment of a spinal prosthesis 100of the present invention. The method 300 will now be discussed withreference to an embodiment of a spinal prosthesis 200. As previouslynoted, once the physician is prepared to implant the prosthesis, he/shecan first estimate the amount of and remove a portion of the vertebralbody, such as facet joints and pedicle, to allow for prosthesisimplantation (310). (FIG. 20 illustrates vertebral bodies 40 and 45after performing one embodiment of a procedural bone resection, a widedecompressive laminectomy, facetectomy and/or laminectomy). In thisembodiment, the spinous process and inferior facet joints have beenremoved from the vertebra 40. The superior facet joints have beenremoved from vertebra 45 and the lamina shaped to produce caudalprosthesis receiving surfaces 74, 72. As illustrated in FIG. 21, holesare formed in the vertebra 40, 45 to prepare for cephalad and caudalstem implantation. Caudal stem holes 52 and 54 are formed in vertebra 45and cephalad stem holes 56, 58 are formed in vertebra 40.

The physician can then size, select, test and set the caudal prosthesis(step 320). As described above, the adaptability of the orientation andposition of the caudal prosthesis may be utilized to meet a wide varietyof anatomical situations. It is to be appreciated that each of theadaptable characteristics of the caudal prosthesis including, forexample, the stem angle θc, the shape of the caudal stem head 162 andthe shape of the caudal cup engaging surface 157 and the lengths of thecaudal stem and distal end may each be used alone or in any combinationto provide caudal stem variability into any orientation sagittally,axially, coronally or combinations thereof. The caudal prosthesis may beconfigured by selecting the desired caudal stem (see stems 160A-160E inFIG. 18), distal tip (if desired—see distal tips 170A-170E) and caudalcup 151 (see FIG. 18). FIG. 22 illustrates the selected caudalcomponents after implantation. The caudal cups 151 are secured to caudalstems (not shown) that have been implanted into the caudal stem holes52, 54 formed in the vertebral body 45.

Size, select, test and adjust the crossbar (step 330). The crossbar 205is selected based on the distance between the caudal cups 151. Thecrossbar may be configured by selecting from a plurality of crossbar 205embodiments each having a different width. Typical fixed width crossbars205 may have a width ranging from 37 to 67 mm, and have a thickness ofapproximately 5 mm and different width increments increasing by 1 or 2mm for each different crossbar. As discussed above, in an alternativeembodiment where adjustable crossbars 205 are used, the width of thecrossbar 205 may be selected initially to place the crossbar ends 215,210 against the caudal cup receiving surface 155. The crossbar width isadjusted into final position using the adjustable crossbar members 104,106 and a threaded portion 109. FIG. 28 illustrates an embodiment of across bar 205 in place against caudal cups 151. The crossbar mounts 275are present with the crossbar 205 disposed within the crossbar engagingportion 274.

Size, select, test and set the cephalad prosthesis (340). Referringinitially to FIG. 28, the cephalad prosthesis 220 is adapted to have thecrossbar engaging end 240 engage with the crossbar cephalad engagingportion 272 and the bone engaging end 125 (not shown) engaged within thelamina via holes 56,58. Within these parameters the cephalad arms 220are configured and adapted by selecting, at least, the desired distaltip 135 length (see distal tips 135A-135E in FIG. 18), and cephalad stem130 length (i.e., cephalad arms 120A-120C in FIG. 18 modified to includean embodiment of the engagement features 242). As described above withcephalad prosthesis elements 120, in some embodiments of the cephaladelements 220, elbow angle and arm length may also be selectablecharacteristics. As illustrated in FIG. 29, the cephalad arms cross barengaging ends 240 are secured by cap 280 and fixation element 282 to thecross bar mount cephalad engaging portion 272. In addition, the cephaladarm 245 has also secured the crossbar relative to crossbar mount 275utilizing the features 242. At the same time, but not illustrated inFIG. 29, tightening the cap 280 also urges the crossbar indexingfeatures 206 into locking cooperation with the crossbar housing indexingfeature 276 to secure the crossbar housing 275 in position betweencrossbar ends 210, 215. Also illustrated is the independence of cephaladarm and crossbar mount adaptability. In this embodiment, it should beappreciated that the cephalad arm end 240 in the crossbar mount adjacentthe end 210 can extend beyond the crossbar mount casing 277 a greater,equal or lesser length than the length the cephalad arm end 240 extendsbeyond the crossbar mount casing 277.

FIGS. 30A and 30B illustrate alternative crossbar mounts 500, 550. Eachof the mounts 500, 550 provide adaptability related to cephalad armheight (“h”), cephalad arm rotation (“r”) and crossbar mount lateralposition (“L”) and rotation. Crossbar and mount 500 includes a centralmount 520 including a pair of articulating mounts 525 shown with aportion of a pair of cephalad arms 545 extending therefrom. While only aportion of the cephalad arms 545 is illustrated, it is to be appreciatedthat the crossbar mounts 500 and 550 function with all of the earlierdescribed embodiments of the adaptable spinal prosthesis described aboveincluding the adaptable caudal and cephalad prosthesis elementembodiments. Moreover, while crossbar 505 is illustrated as a fixedwidth crossbar, it is to be appreciated that the earlier describedadjustable width crossbar concepts may also be applied to embodiments ofcrossbar 505.

The central mount 520 is illustrated in a position between the ends 510,515 and secured to a crossbar 505. The central mount 520 may be adjustedlaterally (“L”) between the ends 510 and 515 by engaging the desired setof complementary engaging elements 504, 506. Engaging elements 504 arelocated within the central mount cross arm engaging portion 574.Engaging elements 506 are located on crossbar 505. The engaging elementsare positioned to allow not only lateral movement but also rotation ofthe central mount about the crossbar 505. Once the central mount is inthe desired position and orientation, the position of the central mount520 relative to the crossbar 505 may be secured using fastener 508.

The central mount 520 includes a pair of articulating mounts 525 thatprovide adaptable, independent cephalad arm height (“h”) and cephaladarm rotation (“r”) for each cephalad arm 545. Separate engaging elementsare provided within a mount 525 and between the mount 525 and centralmount 520 to maintain the desired height and rotation settings for eachcephalad arm 545. Cephalad arm height is maintained by engaging thedesired mount engaging elements 532 with the desired cephalad armengaging elements 530. Once the desired elements are aligned, theengaging elements are locked using the locking element 535. Lockingelement 535 articulates the engaging elements between an “unlocked”configuration and a “locked” configuration. The unlocked configurationis illustrated in the mount 525 adjacent the end 510 and the lockedconfiguration is illustrated in the mount 525 adjacent the end 515.Cephalad arm rotation is achieved by adjusting the position of thearticulating mount engagement elements 534, 536. Engagement elements 534are distributed along the proximate edge of the articulating mount 525.Central mount engagement elements 536 are distributed along the interiorof the central mount 520. Once the desired rotational orientation of thecephalad arm 545 relative to the central mount 520 is achieved, theengagement elements 534, 536 are secured using fastener 538.

An alternative crossbar mount 550 is illustrated in FIG. 30B. Centralmount lateral position (“L”) and rotation operate similar to crossbarmount 500. Crossbar mount 555 includes two internally articulating,lockable cephalad arm mounts 560. The internally articulating, lockablecephalad arm mounts 560 combine the adaptability features of thecephalad arm height (“h”) and cephalad arm rotation (“r”) in a singleadjustment and locking mechanism. The single adjustment and lockingmechanism is provided by a plurality of lockable elements 562 that maybe articulated between and “unlocked” and “locked” configuration by thelocking driver 564. The locking elements are formed from a biocompatiblepolymer or other suitable material to compress against and grip thecephalad arms 545 when secured by the locking driver 564. The lockingdriver 564 may be, in one exemplary embodiment, a band encircling theelements 562 in a first position in an unlocked configuration and in asecond position in the locked configuration. In another exemplaryembodiment, the locking driver is a clamp ring. The elements are shapedwith relative orientations to allow cephalad arm movement to adjust armheight and rotation. When the locking driver 564 is positioned into the“locked” configuration, the elements 562 are gradually engaged so as notto alter the desired height and rotation alignments. The lockingelements 562 are illustrated in an “unlocked” configuration in the mount560 adjacent the end 510. The locking elements 562 are illustrated in a“locked” configuration in the mount 560 adjacent the end 515. The mount560 interior is sized to allow for angular movement and heightadjustments of the cephalad arm 545.

FIGS. 31A to 32D illustrate embodiments an adaptive spinal prosthesis ofthe present invention having alternative embodiments of the crossbarcomponent. These adaptive spinal prosthesis embodiments each includecaudal cups and stems as well as cephalad arms having elbows, stems anddistal ends similar to the earlier described embodiments. For clarity,similar or simplified reference number designations are used todesignate these earlier described components. In addition, thesecomponents will be represented simplistically rather than with fulldetails as before.

FIG. 31A illustrates an embodiment of an adaptive spinal prosthesis 300Ahaving a crossbar 310. The adaptive spinal prosthesis 300A includes apair of cephalad arms each having a cephalad bearing 305 on theproximate end. The crossbar 310 is attached to one cephalad arm 145using fixed clamp 312 and fastener 313. The clamp 312 may be positionedalong the cephalad arm 145 until the clamp 312 and, in turn, thecrossbar 310, is positioned in the desired spacing between the cephaladbearing 305 and the elbow 147. Clamp slide assembly 314 includes apiston 318, joined to clamp 316 using the fastener 315. The clamp slideassembly 314 provides crossbar width adjustment as well as cephaladbearing-elbow spacing for the clamp 316. The clamp 316 secures the clampslide assembly 314 (including the crossbar 310) to the cephalad arm 145in the desired position between the cephalad bearing 305 and elbow 147.The piston 318 is slidably engaged with the crossbar 310. In operationthe piston 318 slides along the crossbar 310 to the desired crossbarwidth. Once the desired crossbar width and cephalad bearing-elbowspacing are obtained, the fastener 315 is tightened. Tightening thefastener 315 secures the crossbar 310 within the piston 318 and thecephalad arm 145 within the clamp 316. In the illustrated embodiment,the clamp 312 and the clamp 316 engage the cephalad arms 145 between thebearing 305 and the elbow 147 in a position leaving the crossbar 310forming an angle of about 90 degrees with each of the cephalad arms 145.It is to be appreciated that the clamps 312, 316 operate independentlyand that the ends of crossbar 310 may attach to the cephalad arms 145 ina configuration where the crossbar 310 forms an angle of other than 90degrees with each of the cephalad arms 145. In the illustratedembodiment, the crossbar 310 lays in a plane below a plane that containsboth cephalad arms 145.

FIG. 31B illustrates an embodiment of an adaptive spinal prosthesis 300Bhaving a telescoping crossbar 310. The telescoping crossbar 320 includestelescoping sections 322 that are attached to clamps 324. Clamps 324 areadjustably engaged about the cephalad arms 145 between the bearing 305and the elbow 147. Fasteners 326 are used to secure the clamps 324 tothe cephalad arms 145. The width of telescoping crossbar 320 may beadjusted as the telescoping sections 322 move towards or away fromfasteners 326. Once the telescoping crossbar 320 width is selected,tightening fasteners 326 secures the crossbar clamps 324 about thecephalad arms 145 and locks the position of the telescoping sections 322in the selected width. In the illustrated embodiment the crossbar 320lies in a plane that contains the cephalad arms 145.

FIG. 31C illustrates an embodiment of an adaptive spinal prosthesis 300Chaving a crossbar embodiment 330 and crossbar locks 331. A crossbar lock331 includes a cephalad arm clamp 334 about a cephalad arm 145 and acrossbar clamp 336 that encircles the crossbar 330. A dual clamp housing332 and fastener 338 join the clamps 334, 336. The width of crossbar 330is determined by moving the crossbar 330 relative to the crossbar clamps336. The crossbar spacing between a cephalad bearing 305 and an elbow147 is determined by moving the cephalad arm clamp 334 along thecephalad arm 147 to the desired position. Once the width of crossbar 330and the position of the crossbar 330 relative to the bearing 305 and theelbow 147 are selected, the crossbar 330 is secured into the selectedposition by tightening the fastener 338. Tightening fastener 338 resultsin articulation within dual clamp housing 332 to tighten both the armclamp 334 about the cephalad arm 145 and the crossbar clamp 336 aboutthe crossbar 330. In the illustrated embodiment the crossbar 330 ispositioned in a plane above a plane that contains the cephalad arms 145,but the crossbar 330 could alternatively be even with or below the planecontaining the cephalad arms 145 (or any combination thereof).

FIG. 31D illustrates an embodiment of an adaptive spinal prosthesis 300Dhaving a crossbar embodiment 340 with crossbar locks 341. A crossbarlock 341 includes a cephalad arm clamp 342, a crossbar clamp 344 and afastener 346. The position of the crossbar 340 between the cephaladbearing 305 and the elbow 147 is changed by sliding the arm clamps 342along the cephalad arms 147. The crossbar width between the crossbarclamps 344 is adjusted by sliding the crossbar 344 relative to theclamps 344. Once the position of the crossbar 340 between the cephaladbearing 305 and the elbow 147 and the width of the crossbar 340 areselected, the crossbar position is secured by tightening fastener 346.Tightening fastener 346 urges the arm clamp 342 about the cephalad arm145 and the crossbar clamp 344 about the crossbar 340. In theillustrated embodiment, the crossbar 340 is contained in a plane above aplane that contains the cephalad arms 145, though it could be even withor below the plane containing the cephalad arms 145, if desired. In theillustrated embodiment, the arm clamp 342 and the crossbar clamp 344 andthe fastener 346 are configured to form a 90 degree angle. Inalternative embodiments of the crossbar 340, angles other than 90degrees may be formed by the crossbar clamp 344, fastener 346 and armclamp 342.

FIG. 31E illustrates an embodiment of an adaptive spinal prosthesis 300Ehaving a crossbar embodiment 350. The crossbar 350 includes a base end352 and an articulating end 353. Each of the base end 352 and thearticulating end 353 include arm clamps 354. Arm clamps 354 are eachsecured to a corresponding cephalad arm 145 by tightening of a set screw357. The articulating end 353 is slidably connected to the base end 352,with the ends 353, 352 similarly secured relative to each other bytightening of a set screw 358. One advantage of the crossbar 350 is thatthe articulating end 353 is free to rotate, telescope and articulateabout the cephalad arm 145 and move relative to the base end 352.

In contrast to attaching the crossbar using a slideable cephalad armclamp attachment as in spinal prosthesis 300A-300E, the following spinalprosthesis embodiments 400A-400D utilize attachment points at oradjacent the cephalad bearing 305. FIG. 32A illustrates an embodiment ofan adaptive spinal prosthesis 400A having a crossbar embodiment 410.Crossbar arm clamps 412 are attached to cephalad bearing 405 using afastener 414 placed into a threaded receiver within cephalad bearing405. Cephalad bearing 405 is threaded to receive fastener 414. When thewidth of the crossbar 410 between the clamps 412 and the cephalad arms145 is in the desired position, the fastener 414 is tightened securingthe clamp 412 about the crossbar 410 and the clamp 412 relative to thecephalad bearing 405. In the illustrated embodiment, the clamps 412 areconfigured to provide the crossbar 410 within a plane that contains thecephalad arms 147. In this specific embodiment, the crossbar 410 lies atapproximately the mid-height of the cephalad arms 147.

FIG. 32B illustrates an embodiment of an adaptive spinal prosthesis 400Bhaving a crossbar embodiment 420 with crossbar clamps 422. Crossbar 420includes clamps 422 that attach about the ends of crossbar 420 and tothe cephalad bearing 405 using fastener 424. The cephalad bearing 405 isdesirably threaded or otherwise configured to receive the fastener 424.The crossbar 420 width is adjustable between the clamps 422. Once thedesired crossbar width is selected, the fastener 424 is tightened. Whenthe fastener 424 is tightened, the clamp 422 secures about the crossbar420 and the clamp 422 is secured relative to the cephalad bearing 405.The cephalad bearing 405 is threaded to receive the fastener 424. In theillustrated embodiment, the clamps 422 are configured such that, whensecured to the cephalad bearings 405, the crossbar 420 is located in aplane above the plane containing the cephalad arms 145 and the clamps422 are positioned between the cephalad arms 145.

FIG. 32C illustrates an embodiment of an adaptive spinal prosthesis 400Chaving a crossbar embodiment 430 with crossbar clamps 432. The crossbarclamps secure the crossbar to the cephalad bearing 405 using thefastener 434. The cephalad bearing 405 is threaded or otherwiseconfigured to receive the fastener 434. In the illustrated embodiment,the crossbar clamps 432 are in line with the cephalad arms 145. Thecrossbar is positioned between the clamps 432 to the desired width. Oncethe crossbar is positioned in the desired width, the fastener 434 istightened. When the fastener 434 is tightened, the clamp 432 is securedabout the crossbar 430 and to the cephalad bearing 405.

FIG. 32D illustrates an embodiment of an adaptive spinal prosthesis 400Dhaving a crossbar embodiment 440 with a locking system 441. A lockingsystem 441 includes a crossbar lock 444 and a cephalad bearing lock 442.Each end of the crossbar 440 is secured to a cephalad bearing 405 usinga locking system 441. Once the width of crossbar 440 between crossbarlocks 444 is selected, then the cephalad bearing lock 442 is pressedinto the cephalad bearing 405. This same motion secures the crossbarlock 444 about crossbar 440 and the bearing lock 442 about the crossbarlock 444.

In alternate embodiments, the crossbar could comprise a plurality ofcrossbars. For example, a first crossbar could fastened to the rightside cephalad bearing with a crossbar attached between the bearing andelbow of the left side cephalad arm. The second crossbar could befastened to the left side cephalad bearing with a crossbar attachedbetween the bearing and elbow of the right cephalad arm. Where the firstand second crossbars cross, they could pass above and below one anotherwithout contact or a bearing/securement surface could be located wherethe first and second crossbars intersect. Alternatively, a pair ofparallel crossbars, either adjacent to one another or spaced apart,connecting the cephalad arms to each other, could be used. Moreover, inembodiments where only a single side of the facet joints in a vertebralbody are replaced, a crossbar could secure the cephalad and/or caudalarms (or both) to the lamina and/or the spinous process. In a similarmanner, the caudal prostheses could incorporate a crossbar or otherarrangement to link the two caudal prostheses together in a like manner.

While the above exemplary adaptive spinal prosthesis and crossbarembodiments have been shown and described with certain features, otherembodiments and alternatives are also within the scope of the invention.For example, the crossbar shape has been illustrated as having acircular or rectangular cross section. Other cross sectional shapes arepossible such as, for example, polygonal, hexagonal, or other suitableshapes. Additionally, crossbar orientation between the crossbar and thecephalad arms has been described as being above, within, or below aplane that contains the cephalad arms 147. It is to be appreciated thateach of the described embodiments may be modified to provide any or allof these crossbar-cephalad arm configurations. Crossbar width may alsobe modified to provide thicknesses and crossbar widths other than thoseillustrated. The crossbar position relative to the cephalad bearing andcephalad arm elbow may also vary from the illustrated embodiments andmay be positioned into configurations below, on top of, or above thecephalad bearing as well as positioned between the cephalad bearing andthe elbow, and including positions adjacent the elbow 147. It is to beappreciated that while each of the above listed crossbar embodiments isillustrated with a straight crossbar, conventional rod bendingtechniques may be utilized to shape the crossbar into a desiredconfiguration further expanding the adaptability aspect of embodimentsof the present invention. In the exemplary embodiments, the clampsjoining the crossbar to the cephalad arms engage the cephalad arms 145in a manner where the crossbar forms an angle of about 90 degrees witheach of the cephalad arms 145. It is to be appreciated that the clampingsystems and elements described herein operate independently and that theends of crossbar may attach to the cephalad arms 145 in alternativeconfigurations, such as, for example, where the crossbar forms an angleof other than 90 degrees with the cephalad arms 145.

Earlier described embodiments of caudal fastener 160 and cephalad boneengaging end 125 have in common a generally linear geometry and similardistal tips 170, 135. However, embodiments of the caudal fastener 160and cephalad bone engaging end 125 may be modified to include one ormore or combinations of anti-rotation and anti-pull out features. Theseadditional features are described below with reference to FIGS. 33A-36C.Irrespective of the design and configuration of the following exemplaryembodiments, the principals illustrated in the embodiments of FIGS.33A-36C are applicable to both caudal and cephalad fasteners even thougha feature or design principal may be shown or described as it may beutilized in either a caudal fastener or a cephalad fastener. Forexample, FIGS. 35A-35D illustrate an anti-rotation paddle in anembodiment of a cephalad prosthesis similar to the cephalad prosthesisillustrated in embodiments of the spinal prosthesis 200 (i.e., FIGS.31A-32D). However, the anti-rotation paddle may be utilized withembodiments of the caudal fastener and/or embodiments of the cephaladelement in spinal prosthesis 100.

FIGS. 33A, 33B, and 33C show an embodiment of a stem 600 with a paddle604 and grooves as anti-rotation element(s). The stem 600 may bemodified to act as a bone engaging end of an embodiment of a cephaladprosthesis element or as a fixation element for an embodiment of acaudal prosthesis. While desiring not to be bound by theory, it isbelieved that the wide surface area(s) provided by the anti-rotationalpaddle embodiments of the present invention provide greater resistanceto the torque loads applied to the prosthesis and attempted rotation ofthe paddle within the vertebra. For example, the addition of surfaceprojections and/or pits can significantly increase the total surfacearea of the prosthesis, thereby increasing the ability of any adhesionbetween the prosthesis and the surrounding material (such as bonecement, epoxy or in-growing bony material) to secure the prosthesis inposition. As another example, the addition of surface projections andpits can interact with the surrounding material to create a geometric ormechanical “interlock” that resists relative motion between theprosthesis and the surrounding material. As such, the paddle embodimentsof the present invention described herein act as improvedanti-rotational and/or anti pull-out elements. Similarly, otheranti-rotation elements described herein are also used to counteract thetorque and/or axial loads developed within and acting upon variousportions of vertebral prosthesis.

The stem 600 has a distal end 601 and a proximal end 602. The proximalend 602 may be configured to accept tooling and instruments to securethe stem 600 into the vertebra and/or to provide an attachment point toanother component within an embodiment of an adaptable spinal prosthesisof the present invention. The distal portion of the stem 600 includes apaddle 604 configured to act as an anti-rotation element to prevent therotation of the stem 600 once implanted into a portion of the spine.Alternative embodiments of the stem 600 can have multiple paddles.Although the illustrated paddle 604 has a rounded profile, alternativeembodiments may have different profiles including, for example, one ormore corners. Although the illustrated paddle 604 is flat, alternativeembodiments can have nonflat contours, with one or more concave and/orconvex features.

FIGS. 33A, 33B, and 33C also illustrate an embodiment of an anti-pullout feature of the stem 600. Embodiments of the stem 600 also includeanti-pull out features. As used herein, an anti-pull out feature refersto an element or combination of elements of a prosthesis portion orfastener acting to mitigate, minimize or counteract forces bearing uponthe prosthesis portion or fastener to disengage, loosen, advance, pullor otherwise axially translate the fastener relative to a desiredposition on or within the vertebra. (For purposes of this disclosure,anti-pullout forces can be interpreted to include, but are not limitedto, both “pull” and “push” forces, as well as components of varioustwisting and/or rotational forces, which serve to translate theprosthesis along a longitudinal axis outward or inward relative to thetargeted vertebral body.) In the illustrated embodiment, the stem 600includes a proximal grooved portion 605 having proximal grooves 606 anda distal grooved portion 615 having distal grooves 617. In theillustrated embodiment, proximal grooves 606 have a proximal tip with awidth that increases distally and distal grooves 617 have a nearlyconstant width terminating in a distal tip. A reduced diameter portion608 separates the proximal grooved portion 605 from the distal groovedportion 615. The proximal grooves 606, distal grooves 617 and reduceddiameter section 608 act to increase the surface area of the vertebralprosthesis portion 600. Increasing the surface area of the stem 600provides greater attachment between the stem 600 and the vertebra. Thegreater amount of surface area may be used advantageously with bonecement, bone growth compounds or other materials used to bond theexternal surfaces the stem 600 to the interior of the vertebra. Thegreater surface area allows, in embodiments where bone fixation cementis used, more cement to be present along the length and a particularlygreater amount of cement or fixation material to be present about thereduced diameter section 608. The increased amount of cement presentadjacent the reduced diameter portion 608 (and increased thickness ofthe cement mantle in these areas) produces a section of increaseddiameter that strengthens the overall mantle and/or counteracts pull outforces. Other configurations, arrangements and geometries of theproximal grooved portion 605, reduced diameter portion 608, and distalgrooved portion 615 are possible. For example, different grooveconfigurations are possible (e.g., FIGS. 34A, 34B, 36A, 36B and 36C),there may be multiple distal or proximate grooved portions, multiplereduced diameter portions or different paddle configurations (e.g.,FIGS. 35A-35D).

FIGS. 34A and 34B illustrate an alternative embodiment of stems 900, 990having anti-rotation and anti-pullout elements. The paddle 955 andproximal ridges 925, 927 act as anti-rotation elements. The reduceddiameter section 940, grooved sections 930, 945 and reduced shankdiameter 920, 922 act as anti-pullout elements. The stems 900 and 990are similar in many regards to stem 600 of FIGS. 33A, 33B and 33C.However, several differences are worth noting. Paddle 955 has a flatface 960 but a rounded, tapered distal end 965 instead of a flat distaledge found on paddle 604 (see FIG. 33B). Proximal grooves 935 have aconstant width instead of a tapered width (see FIG. 33A grooves 606).Distal grooves 950 have a uniform width and a rounded distal end insteadof a distal tip (grooves 617 of FIG. 33B).

One notable difference between the stems 900, 990 and the stem 600 isthe addition of the proximal anti-rotation sections 920, 922. Theproximal anti-rotation sections 920, 922 include a shank having adiameter less than the shank 915 and a plurality (two in the illustratedembodiments) of ridges that act as proximal anti-rotation elements. Stem900 has a proximal anti-rotation portion 920 and ridges 925 having anoverall height h1. Stem 990 has a proximal anti-rotation portion 922 andridges 927 having an overall height h2. These embodiments advantageouslyprovide reduced shank sizes thereby allowing for increased cement mantle(if cement is desired), while still providing a mechanical “interlock”with the surrounding tissue that resists prosthesis rotation—in variousembodiments, the ridges can desirably engage surrounding cortical boneat the pedicle entry point, which is often stronger than the cancellousbone contained within the vertebral body, although the ridges'engagement with either or both types of bone will serve to resistrotation to varying degrees. In a specific embodiment of the stem 900the height h₁ is 8.25 mm and the proximal anti-rotation section diameteris 6.5 mm but still desirably maintains a moment of inertia (ly)approximately equal to that of a 7 mm rod. In a specific embodiment ifthe stem 990, the overall ridge height h2 is 8.75 mm and the proximalanti-rotation section diameter is 6.0 mm but the embodiment stilldesirably maintains a moment of inertia (ly) approximately equal to thatof a 7 mm rod.

It is to be appreciated that the stems 900, 990 may differ from theillustrated embodiments. For example, there may be one or more ridgespresent in the proximal anti-rotation sections (as opposed to the pairof ridges disclosed above). The additional ridges need not have uniformcross sections or be uniformly spaced about the perimeter of theproximal anti-rotation section. The paddle face 960 may have a differentface such as convex, concave or other compound shape or combinationsthereof.

FIGS. 35A-D show an embodiment of a cephalad arm 700 with a fixationelement having a bend 710, and a paddle 704 as an anti-rotation element,similar to the stem 600 of FIGS. 6A, 6B, and 6C. The cephalad arm 700includes a distal end 701 and a proximal end 703. The proximal end 703includes a bearing element 715 for engagement to other portions of thevertebral prosthesis. To accommodate a number of different facet jointprosthesis configurations, the fixation element includes a bend 710connected to a shaft 735 having a paddle 704 attached thereto.

The cephalad arm 700 also illustrates another aspect of the adaptableand configurable concepts of the present invention. For example, in someembodiments, the shaft 735 is detachably fastened to the attachmentpoint 740. The shaft 735 has a length “l” between the attachment point740 and the proximate end of the paddle 704. The shaft 735 is detachablycoupled to the attachment point 740 to allow for shafts 735 of differentlengths to be used with different configurations of the cephalad arm 700thereby providing a modular vertebral prosthesis. As such, in use, theshaft 735 may be detached from the attachment point 740 and replacedwith a shaft 735 having a different length “l” as needed until theproper alignment of the vertebral prosthesis is achieved. The highlyconfigurable and modular components of embodiments of the spinalprosthesis of the present invention can be attached to the prosthesisusing one or more attachments methods well known in the art, includingthreaded screws, Morse (or other types) tapers, welding, adhesives orset screws.

While the modular concept has been described with regard to thevertebral prosthesis 700, it is to be appreciated that other Embodimentsof the cephalad arm 700 described herein may have a portion or portionsthat are detachably coupled in furtherance of the configurable,adaptable spinal prosthesis concepts of the present invention. For analternative example, the shaft 735 may be of fixed length andpermanently attached to the attachment point 740 while the detachableattachment point is positioned between the shaft 735 and the paddle 704thereby allowing paddles 704 of different lengths to be used. In yetanother alternative, both the shaft and the paddle may have detachableattachment points thereby allowing various shaft lengths andconfigurations and paddle lengths and configurations to be used infurtherance of the modular spinal prosthesis concepts described herein.It is to be appreciated that the detachable attachment point may bepositioned between any portion or portions of the embodiments of thespinal prosthesis portions described herein. Similarly, the anchoringdevices may comprise pedicles screws or other similar modules whichprovide a solid anchor to the vertebral body, which can in turn beattached to various modules that either (1) replace the facet jointstructure (allowing for motion) or (2) immobilize the facet jointstructure (as an adjunct to spinal and/or facet joint fusion).

In an alternate embodiment, one or more sections of the stem or cephaladarm prosthesis may be made of a deformable or shape-memory material(such as Nitinol or similar materials), which permits the physician tomake adjustments to the prosthesis geometry to “form-fit” the implant tothe patient's specific anatomy. In the case of Nitinol, the material canbe heated or cooled away from the body temperature (depending upon thetype of material and it's martensitic/austenitic properties), bedeformed to a desired shaped, and then held in the deformed position andallowed to return to the body temperature, thereby “hardening” into thedesired shape or form. Such an embodiment would facilitate a reductionin the number of sections or “modules” required for a modularprosthesis, as each module could assume a variety of desired positions.

While the angle of the illustrated bend 710 is acute, other embodimentsof the cephalad arm 700 can have a bend 710 having a right angle or anobtuse angle. Alternative embodiments of the cephalad arm 700 mayinclude two, three, or more bends 710. In the illustrated embodiment,the paddle 704 has a flat surface 720 and a proximal end having atransition portion 730. The flat surface 720 is illustrated in the sameplane in which the fixation element has the bend 710. In otherembodiments, the paddle 704 has a flat surface 720 in another plane,and/or a nonflat contour, with one or more concave and/or convexfeatures or have paddle shapes (the flat surface 720 can be at virtuallyany angle relative to the angle of the elbow, including perpendicular toor parallel to the bend 710). The transition portion 730 has a widththat decreases linearly in a proximal direction. Other configurations ofthe transition portion 730 are possible for transitioning from thepaddle 704 to the shaft 735 of the vertebral prosthesis portion 700. Thealternative shapes of the transition portion include, for example, anon-linear decreasing proximal width, asymmetric portions, curvedportions or compound portions.

FIGS. 36A and 36B show an embodiment of a cephalad arm 1400 with helicallongitudinal depressions as anti-rotation elements and a fixationelement with a bend. The illustrated embodiment of the cephalad arm 1400has a distal tip 1404 and a proximal end 1402. The proximal end 1402includes a socket element 1407 for further attachment to a vertebralprosthesis component. In an alternative embodiment, the element 1407could comprise a cephalad bearing surface for slidably engaging acorresponding caudal cup as described above with regard to an embodimentof a spinal prosthesis of the present invention. Proximal shaft 1415 isattached to the socket element 1407 and the bend 1410. The taperedsection 1430 transitions from the proximal shaft 1415 to the distalshaft 1417. The proximal shaft 1415 is a different diameter than thedistal shaft 1417. Other transitions are possible such as a steppedtransition (e.g. section 740 of FIG. 35B) or no transition if thediameter of the shafts 1415 and 1417 are the same.

The distal shaft 1417 includes a plurality of longitudinal depressions1423 extending from the distal end 1404 to a point beyond the taperedsection 1430. The proximal end of the longitudinal depressions 1423 hasa bulbed section 1460. The distal shaft 1417 also includes a reduceddiameter section 1440. The reduced diameter section 1440, longitudinalgrooves 1423 and bulbed section 1460 may be used to increase the surfacearea of the vertebral prosthesis portion 1440 that is, when implanted,within a vertebra of the spine. The increased surface area allows formore area to support the cement mantle for applications using cement orbony in-growth for applications using bone ingrowth. It is to beappreciated that the longitudinal grooves 1423 may also be varied asdescribed elsewhere with regard to other grooves and, for example, asdescribed with regard to FIGS. 33A-33C. In addition, alternativeembodiments of bend 1410 are possible as described with regard to FIGS.35A-35D.

It is to be appreciated that each of the longitudinal depressions 1423has a longitudinally varying profile, narrowing as the longitudinaldepression extends proximally. In alternative embodiments, thelongitudinally varying profile can widen or remain constant as thelongitudinal depression extends proximally. Although in the illustratedembodiment all of the longitudinal depressions are identical, in otherembodiments, the multiple longitudinal depressions can differ, forexample by having different profiles, lengths, starting and/or endingpoints, etc. Alternative embodiments can have one longitudinaldepression, two longitudinal depressions, four longitudinal depressions,five longitudinal depressions, or more longitudinal depressions.

FIG. 36C depicts an alternate embodiment of the vertebral prosthesis ofFIG. 36A, 36B in which a pair of cephalad prosthesis arms 1400 areconnected by a cross-bar 1405. The crossbar 1405 provides yet anotheralternative arm attachment in addition to the crossbar-cephalad armattachment embodiments illustrated in FIGS. 31A-32D. Cross-bar 1405 canbe a cylindrical member fitting into openings 1409 in each of the shafts1415 of the prosthesis arms 1400 (or can be virtually any rigid orsemi-rigid member secured between the two prosthesis arms), and thecross-bar 1405 desirably reduces or prevents rotation of the prosthesisarms 1400 relative to each other. When both of the prosthesis arms aresecured into a targeted vertebral body through the pedicles (not shown),any torsional loads experienced by an individual prosthesis arm 1400will be transferred to the shaft 1415 of the opposing prosthesis arm bythe cross-bar 1405, which will convert the torsional load to atransverse load acting on the opposing prosthesis. Desirably, the newlyloaded prosthesis arm can resist this transverse force, therebymaintaining the entire structure in a desired position. In thisembodiment, the cross-bar therefore “shares” and redistributes thetorsional loading experienced by an individual prosthesis arm,significantly reducing the tendency for an individual prosthesis arm todeflect and/or rotate. In an alternative embodiment the crossbar 1405may have an adjustable portion that allows adjustment in the widthbetween the cephalad prosthesis arms 1400.

Additional anti-pull out and anti-rotation embodiments and disclosuresare described in commonly assigned U.S. patent application to Tokish etal. entitled “Anti-Rotation Fixation Element for Spinal Prostheses,”Ser. No. 10/831,657, filed Apr. 22, 2004, the entirety of which isincorporated herein by reference for all purposes.

Additional trialing embodiments and disclosures are described incommonly assigned U.S. patent application to Augostino et al entitled“Facet Joint Prosthesis Measurement and Implant Tools,” Ser. No.10/831,651, filed Apr. 22, 2004, the entirety of which is incorporatedherein by reference for all purposes.

In further embodiments, one or more surfaces of the embodiments of thespinal prosthesis of the invention may be covered with various coatingssuch as antimicrobial, antithrombotic, and osteoinductive agents, or acombination thereof (see, e.g., U.S. Pat. No. 5,866,113, which isincorporated herein by reference). These agents may further be carriedin a biodegradable carrier material with which the pores of the stemand/or cup member of certain embodiments may be impregnated (see, e.g.,U.S. Pat. No. 5,947,893, which is also incorporated herein byreference).

While the above described embodiments have been shown and describedutilizing a crossbar having two ends and pairs of cephalad and caudalprosthesis elements, it is to be appreciated that embodiments of thepresent invention may include adaptable spinal prosthesis embodimentsutilizing the inventive concepts described herein for a single cephaladelement, single caudal element and a crossbar having only one end.

What is claimed is:
 1. A surgical method comprising: providing a spinalprosthesis to replace a portion of the spine, the spinal prosthesiscomprising: a central mount; first and second articulating mountscoupled to the central mount, the first articulating mount includes aportion of a first cephalad arm extending therefrom and the secondarticulating mount includes a portion of a second cephalad arm extendingtherefrom; a crossbar configured to be secured to the central mount;wherein the central mount is configured to be adjusted laterally alongthe crossbar, wherein the first and second articulating mounts areconfigured to be adjustable independently of each other; and implantingthe spinal prosthesis into a portion of the spine.
 2. The surgicalmethod of claim 1, wherein the crossbar comprises a two-piece bar. 3.The surgical method of claim 1, wherein the spinal prosthesis comprisesa caudal prosthesis element.
 4. The surgical method of claim 1, whereinthe first and second articulating mounts each comprises a cephalad armengaging portion.
 5. The surgical method of claim 1, wherein the centralmount is adjustable rotationally about the crossbar.
 6. The surgicalmethod of claim 1, wherein the spinal prosthesis is implanted into atleast one vertebral body.
 7. The surgical method of claim 1 furthercomprising, before implanting the spinal prosthesis, forming one or morestem holes in a vertebral body, and subsequently, implanting the spinalprosthesis by inserting a bone engaging end extending from each of thefirst and second cephalad arms into the respective stem holes.
 8. Asurgical method comprising: providing a spinal prosthesis to replace aportion of the spine, the spinal prosthesis comprising: a central mountcoupled to a crossbar configured to extend across a vertebra; first andsecond articulating mounts coupled to the central mount, the firstarticulating mount includes a portion of a first cephalad arm extendingtherefrom and the second articulating mount includes a portion of asecond cephalad arm extending therefrom: locking elements for securingthe first and second articulating mounts; wherein the central mount isconfigured to be adjusted laterally along the crossbar, wherein thefirst and second articulating mounts are configured to be adjustableindependently of each other, wherein the crossbar replaces a portion ofa first and second facet joint; and implanting the spina1 prosthesisinto a portion of spine.
 9. The method of claim 8, wherein the centralmount is configured to be adjusted laterally between a first end and asecond end of the crossbar.
 10. The method of claim 9, wherein thecentral mount includes a first engaging element and the crossbarincludes a second engaging element complementary to the first engagingelement.
 11. The method of claim 9, wherein the central mount isadjustable rotationally about the crossbar.
 12. The method of claim 8,wherein the central mount is configure to be adjusted laterally betweena first and second end of the crossbar.
 13. The method of implant forstabilizing the spine as in claim 8, wherein the central mount includesa first engaging element and the crossbar includes a second engagingelement complementary to the first engaging element.
 14. A surgicalmethod comprising: providing a spinal prosthesis to replace a portion ofthe spine, the spinal prosthesis comprising: a central mount coupled toa crossbar configured to extend across a vertebra; a first and secondarticulating mounts coupled to the central mount, the first articulatingmount includes a portion of a first cephalad arm extending therefrom andthe second articulating mount includes a portion of a second cephaladarm extending therefrom: wherein the first and second articulatingmounts are configured to be adjustable independently of each other,wherein the crossbar replaces a portion of a first and second facetjoint, wherein the central mount is adjustable rotationally about thecrossbar; and implanting the spinal prosthesis into a portion of thespine.
 15. The surgical method of claim 14, wherein the crossbarcomprises a two-piece bar.
 16. The surgical method of claim 14, whereinthe spinal prosthesis comprises a caudal prosthesis element.
 17. Thesurgical method of claim 14, wherein the first and second articulatingmounts each comprises a cephalad arm engaging portion.
 18. The surgicalmethod of claim 14, wherein the central mount is adjustable laterallyabout the crossbar.